Repl with let statements mostly working.

dm/src/main.rs

@@ -4,6 +4,7 @@ mod run;
 use crate::repl::repl;
 use crate::run::compile_and_run_script;
 use clap::{Parser, Subcommand};
+use std::io;
 use std::path::PathBuf;
 
 #[derive(Debug, Parser)]
@@ -23,29 +24,27 @@ enum SubCommand {
 
         #[arg(long)]
         show_ir: bool,
-
-        #[arg(long, default_value = "8")]
-        register_count: usize,
-    },
-    Repl {
-        #[arg(long, default_value = "8")]
-        register_count: usize,
     },
+    Repl,
 }
 
 fn main() {
+    let register_count = std::env::var("DVM_REGISTER_COUNT")
+        .map(|v| v.parse::<usize>())
+        .unwrap_or(Ok(8))
+        .unwrap();
+
     let args = Cli::parse();
     match &args.sub_command {
         SubCommand::Run {
             script,
             show_asm,
             show_ir,
-            register_count,
         } => {
-            compile_and_run_script(script, *show_asm, *show_ir, *register_count);
+            compile_and_run_script(script, *show_asm, *show_ir, register_count);
         }
-        SubCommand::Repl { register_count } => {
-            repl(*register_count);
+        SubCommand::Repl => {
+            repl(&mut io::stdin().lock(), register_count);
         }
     }
 }

dm/src/repl.rs

@@ -4,28 +4,44 @@ use dmc_lib::diagnostic::Diagnostic;
 use dmc_lib::ir::ir_function::IrFunction;
 use dmc_lib::ir::ir_return::IrReturn;
 use dmc_lib::ir::ir_statement::IrStatement;
+use dmc_lib::ir::ir_variable::IrVariable;
 use dmc_lib::lexer::Lexer;
-use dmc_lib::parser::parse_expression;
+use dmc_lib::offset_counter::OffsetCounter;
+use dmc_lib::parser::{parse_expression, parse_let_statement};
+use dmc_lib::symbol::variable_symbol::VariableSymbol;
 use dmc_lib::symbol_table::SymbolTable;
 use dmc_lib::token::TokenKind;
+use dmc_lib::type_info::TypeInfo;
 use dmc_lib::types_table::TypesTable;
 use dvm_lib::vm::constant::{Constant, StringConstant};
 use dvm_lib::vm::function::Function;
 use dvm_lib::vm::operand::Operand;
-use dvm_lib::vm::{CallStack, DvmContext, call};
+use dvm_lib::vm::{CallStack, DvmContext, loop_instructions, prepare_for_instruction_loop};
+use std::cell::RefCell;
+use std::collections::HashMap;
 use std::io;
-use std::io::Write;
+use std::io::{BufRead, Write};
+use std::rc::Rc;
 
-pub fn repl(register_count: usize) {
+pub fn repl(read: &mut impl BufRead, register_count: usize) {
     let mut buffer = String::new();
 
+    let mut symbol_table = SymbolTable::new();
+    let mut types_table = TypesTable::new();
+
+    let mut repl_fn_body_scope_id: Option<usize> = None;
+    let mut repl_fn_offset_counter = OffsetCounter::new();
+    let mut repl_fn_local_variables = HashMap::new();
+
     let mut constants_table = ConstantsTable::new();
     let mut context = DvmContext::new();
 
+    let mut repl_fn_stack_locals: Vec<Operand> = vec![];
+
     'repl: loop {
         print!("> ");
         io::stdout().flush().unwrap();
-        io::stdin().read_line(&mut buffer).unwrap();
+        read.read_line(&mut buffer).unwrap();
         let input = buffer.trim();
         if input.is_empty() {
             buffer.clear();
@@ -52,9 +68,40 @@ pub fn repl(register_count: usize) {
                 todo!("Parse functions in repl")
             }
             TokenKind::Let => {
-                todo!("Parse let statements in repl")
+                match compile_let_statement(
+                    input,
+                    register_count,
+                    &mut symbol_table,
+                    &mut repl_fn_body_scope_id,
+                    &mut repl_fn_offset_counter,
+                    &mut repl_fn_local_variables,
+                    &mut types_table,
+                    &mut constants_table,
+                ) {
+                    Ok(function) => {
+                        context
+                            .functions_mut()
+                            .insert(function.name_owned(), function);
                     }
-            _ => match compile_expression(input, register_count, &mut constants_table) {
+                    Err(diagnostics) => {
+                        for diagnostic in diagnostics {
+                            eprintln!("{}", diagnostic.message());
+                        }
+                        buffer.clear();
+                        continue 'repl;
+                    }
+                }
+            }
+            _ => match compile_expression(
+                input,
+                register_count,
+                &mut symbol_table,
+                &mut types_table,
+                &mut repl_fn_body_scope_id,
+                &mut repl_fn_local_variables,
+                &mut repl_fn_offset_counter,
+                &mut constants_table,
+            ) {
                 Ok(function) => {
                     context
                         .functions_mut()
@@ -63,10 +110,10 @@ pub fn repl(register_count: usize) {
                 Err(diagnostics) => {
                     for diagnostic in &diagnostics {
                         eprintln!("{}", diagnostic.message());
+                    }
                     buffer.clear();
                     continue 'repl;
                 }
-                }
             },
         }
 
@@ -77,10 +124,25 @@ pub fn repl(register_count: usize) {
             );
         }
 
-        let mut registers = vec![Operand::Null; register_count];
         let mut call_stack = CallStack::new();
+        prepare_for_instruction_loop(&context, "__repl", &mut call_stack, &[]);
+
+        // copy all old locals to current call frame's stack
+        // this has to be done with indexing because the preparation above creates space for them
+        for (i, operand) in repl_fn_stack_locals.iter().enumerate() {
+            let target_index = call_stack.top().fp() + i;
+            call_stack.top_mut().stack_mut()[target_index] = operand.clone();
+        }
+
+        let result = loop_instructions(
+            &context,
+            &mut vec![Operand::Null; register_count],
+            &mut call_stack,
+        );
+
+        // copy the top frame's stack locals back to OUR stack locals for next iteration
+        repl_fn_stack_locals = std::mem::take(call_stack.top_mut().stack_mut());
 
-        let result = call(&context, &mut registers, &mut call_stack, "__repl", &[]);
         if let Some(value) = result {
             println!("{}", value);
         }
@@ -89,35 +151,58 @@ pub fn repl(register_count: usize) {
     }
 }
 
+fn prepare_scopes(symbol_table: &mut SymbolTable, fn_body_scope_id: &mut Option<usize>) -> usize {
+    if let Some(scope_id) = fn_body_scope_id {
+        symbol_table.change_scope(*scope_id);
+        *scope_id
+    } else {
+        symbol_table.push_module_scope("__repl_module");
+        symbol_table.push_function_scope("__repl_fn");
+        let container_scope_id = symbol_table.push_block_scope("__repl_fn_body");
+        fn_body_scope_id.replace(container_scope_id);
+        container_scope_id
+    }
+}
+
 fn compile_expression(
     input: &str,
     register_count: usize,
+    symbol_table: &mut SymbolTable,
+    types_table: &mut TypesTable,
+    fn_body_scope_id: &mut Option<usize>,
+    fn_local_variables: &HashMap<Rc<VariableSymbol>, Rc<RefCell<IrVariable>>>,
+    offset_counter: &mut OffsetCounter,
     constants_table: &mut ConstantsTable,
 ) -> Result<Function, Vec<Diagnostic>> {
-    let parse_result = parse_expression(input);
-    let mut expression = parse_result?;
+    // parse
+    let mut expression = parse_expression(input)?;
 
-    let mut symbol_table = SymbolTable::new();
-    let mut types_table = TypesTable::new();
+    // init scopes, if necessary
+    let container_scope = prepare_scopes(symbol_table, fn_body_scope_id);
 
-    // "fake" scopes
-    symbol_table.push_module_scope("__repl_module");
-    let function_scope_id = symbol_table.push_function_scope("__repl_function");
-
-    expression.init_scopes(&mut symbol_table, function_scope_id);
-
-    symbol_table.pop_scope(); // function
-    symbol_table.pop_scope(); // module
+    // inner scopes
+    expression.init_scopes(symbol_table, container_scope);
 
+    // names
     let diagnostics = expression.check_static_fn_local_names(&symbol_table);
     if !diagnostics.is_empty() {
         return Err(diagnostics);
     }
 
-    expression.type_check(&symbol_table, &mut types_table)?;
+    // type check
+    expression.type_check(&symbol_table, types_table)?;
 
     // synthesize a function
+    // init ir_builder
     let mut ir_builder = IrBuilder::new();
+
+    // copy all previous declared variables to here so we preserve their stack offsets
+    for (key, value) in fn_local_variables {
+        ir_builder
+            .local_variables_mut()
+            .insert(key.clone(), value.clone());
+    }
+
     let entry_block_id = ir_builder.new_block();
 
     let maybe_ir_expression =
@@ -138,6 +223,75 @@ fn compile_expression(
         entry_block.clone(),
     );
 
-    let (_, stack_size) = ir_function.assign_registers(register_count);
-    Ok(ir_function.assemble(stack_size, constants_table))
+    // spilled registers are put on the stack, so we need to add it to our stack size
+    ir_function.assign_registers(register_count, offset_counter);
+
+    Ok(ir_function.assemble(offset_counter.get_count(), constants_table))
+}
+
+fn compile_let_statement(
+    input: &str,
+    register_count: usize,
+    symbol_table: &mut SymbolTable,
+    body_scope_id: &mut Option<usize>,
+    offset_counter: &mut OffsetCounter,
+    local_variables: &mut HashMap<Rc<VariableSymbol>, Rc<RefCell<IrVariable>>>,
+    types_table: &mut TypesTable,
+    constants_table: &mut ConstantsTable,
+) -> Result<Function, Vec<Diagnostic>> {
+    // parse
+    let mut let_statement = parse_let_statement(input)?;
+
+    // names
+    let container_scope_id = prepare_scopes(symbol_table, body_scope_id);
+
+    let_statement.init_scopes(symbol_table, container_scope_id);
+
+    let name_diagnostics = let_statement.analyze_static_fn_local_names(symbol_table);
+    if !name_diagnostics.is_empty() {
+        return Err(name_diagnostics);
+    }
+
+    // types
+    let_statement.type_check(&symbol_table, types_table)?;
+
+    // init the ir builder
+    let mut ir_builder = IrBuilder::new();
+
+    // put previous locals in ir_builder so expressions can find them
+    for (k, v) in local_variables.iter() {
+        ir_builder
+            .local_variables_mut()
+            .insert(k.clone(), v.clone());
+    }
+
+    // ir function
+    let entry_block_id = ir_builder.new_block();
+
+    let destination_ir_variable = let_statement.to_repl_ir(
+        &mut ir_builder,
+        symbol_table,
+        types_table,
+        offset_counter.next() as isize,
+    ); // put it on top
+
+    // Now that we've translated to ir, we can add the new local variable in the IrBuilder to our
+    // record of them to be used for next loop iteration.
+    let variable_symbol = let_statement.get_destination_symbol(symbol_table);
+    local_variables.insert(variable_symbol, destination_ir_variable);
+
+    ir_builder.finish_block();
+    let entry_block = ir_builder.get_block(entry_block_id);
+    let mut ir_function = IrFunction::new(
+        "__repl".into(),
+        vec![],
+        &TypeInfo::Void,
+        entry_block.clone(),
+    );
+
+    // By here, the variables should all be assigned to their new or existing stack slots.
+    // Register allocation should only be required for temp variables.
+    ir_function.assign_registers(register_count, offset_counter);
+
+    Ok(ir_function.assemble(offset_counter.get_count(), constants_table))
 }

dm/src/run.rs

@@ -6,13 +6,13 @@ use dm_std_lib::add_all_std_core;
 use dmc_lib::constants_table::ConstantsTable;
 use dmc_lib::diagnostic::Diagnostic;
 use dmc_lib::intrinsics::{insert_intrinsic_symbols, insert_intrinsic_types};
+use dmc_lib::offset_counter::OffsetCounter;
 use dmc_lib::parser::parse_compilation_unit;
 use dmc_lib::symbol_table::SymbolTable;
 use dmc_lib::types_table::TypesTable;
 use dvm_lib::vm::constant::{Constant, StringConstant};
 use dvm_lib::vm::function::Function;
-use dvm_lib::vm::operand::Operand;
-use dvm_lib::vm::{CallStack, DvmContext, call};
+use dvm_lib::vm::{DvmContext, call};
 use std::path::PathBuf;
 use std::rc::Rc;
 
@@ -67,8 +67,9 @@ fn run(
     let mut constants_table = ConstantsTable::new();
 
     for ir_function in &mut ir_functions {
-        let (_, stack_size) = ir_function.assign_registers(register_count);
-        let function = ir_function.assemble(stack_size, &mut constants_table);
+        let mut offset_counter = OffsetCounter::new();
+        ir_function.assign_registers(register_count, &mut offset_counter);
+        let function = ir_function.assemble(offset_counter.get_count(), &mut constants_table);
         functions.push(function);
     }
 
@@ -107,16 +108,8 @@ fn run(
         );
     }
 
-    let mut registers: Vec<Operand> = vec![Operand::Null; register_count];
-    let mut call_stack = CallStack::new();
+    let result = call(&dvm_context, "main", &vec![], register_count);
 
-    let result = call(
-        &dvm_context,
-        &mut registers,
-        &mut call_stack,
-        "main",
-        &vec![],
-    );
     if let Some(value) = result {
         println!("{}", value);
     }

dmc-lib/src/ast/compilation_unit.rs

@@ -8,7 +8,7 @@ use crate::ir::ir_class::IrClass;
 use crate::ir::ir_function::IrFunction;
 use crate::symbol_table::SymbolTable;
 use crate::types_table::TypesTable;
-use crate::{diagnostics_result, handle_diagnostics, ok_or_err_diagnostics};
+use crate::{diagnostics_result, handle_diagnostics};
 
 pub struct CompilationUnit {
     functions: Vec<Function>,

dmc-lib/src/ast/let_statement.rs

@@ -10,6 +10,7 @@ use crate::symbol::Symbol;
 use crate::symbol::class_symbol::ClassSymbol;
 use crate::symbol::variable_symbol::VariableSymbol;
 use crate::symbol_table::SymbolTable;
+use crate::type_info::TypeInfo;
 use crate::types_table::TypesTable;
 use std::cell::RefCell;
 use std::rc::Rc;
@@ -55,6 +56,10 @@ impl LetStatement {
         self.initializer.init_scopes(symbol_table, container_scope);
     }
 
+    pub fn scope_id(&self) -> usize {
+        self.scope_id.unwrap()
+    }
+
     fn make_and_insert_variable_symbol(
         &self,
         symbol_table: &mut SymbolTable,
@@ -131,6 +136,7 @@ impl LetStatement {
         types_table: &mut TypesTable,
     ) -> Result<(), Vec<Diagnostic>> {
         self.initializer.type_check(symbol_table, types_table)?;
+        // TODO: this is wrong. We need to check assignability
         let initializer_type_info = self
             .initializer
             .type_info(symbol_table, types_table)
@@ -144,6 +150,34 @@ impl LetStatement {
         Ok(())
     }
 
+    fn make_vr_variable(&self, builder: &mut IrBuilder, destination_type: &TypeInfo) -> IrVariable {
+        IrVariable::new_vr(
+            self.declared_name().into(),
+            builder.current_block().id(),
+            destination_type,
+        )
+    }
+
+    fn make_stack_variable(
+        &self,
+        builder: &mut IrBuilder,
+        destination_type: &TypeInfo,
+        offset: isize,
+    ) -> IrVariable {
+        IrVariable::new_stack_with_offset(
+            self.declared_name().into(),
+            builder.current_block().id(),
+            destination_type,
+            offset,
+        )
+    }
+
+    pub fn get_destination_symbol(&self, symbol_table: &SymbolTable) -> Rc<VariableSymbol> {
+        symbol_table
+            .get_variable_symbol_owned(self.scope_id.unwrap(), &self.declared_name)
+            .unwrap()
+    }
+
     pub fn to_ir(
         &self,
         builder: &mut IrBuilder,
@@ -154,20 +188,14 @@ impl LetStatement {
             .initializer
             .to_ir_operation(builder, symbol_table, types_table);
 
-        let destination_symbol = symbol_table
-            .get_variable_symbol_owned(self.scope_id.unwrap(), &self.declared_name)
-            .unwrap();
+        let destination_symbol = self.get_destination_symbol(symbol_table);
 
         let destination_type = types_table
             .variable_types()
             .get(&destination_symbol)
             .unwrap();
 
-        let destination_vr_variable = IrVariable::new_vr(
-            self.declared_name().into(),
-            builder.current_block().id(),
-            destination_type,
-        );
+        let destination_vr_variable = self.make_vr_variable(builder, destination_type);
 
         let as_rc = Rc::new(RefCell::new(destination_vr_variable));
         let ir_assign = IrAssign::new(as_rc.clone(), init_operation);
@@ -180,4 +208,30 @@ impl LetStatement {
             .current_block_mut()
             .add_statement(IrStatement::Assign(ir_assign));
     }
+
+    pub fn to_repl_ir(
+        &self,
+        builder: &mut IrBuilder,
+        symbol_table: &SymbolTable,
+        types_table: &TypesTable,
+        destination_stack_offset: isize,
+    ) -> Rc<RefCell<IrVariable>> {
+        let init_operation = self
+            .initializer
+            .to_ir_operation(builder, symbol_table, types_table);
+        let destination_symbol = self.get_destination_symbol(symbol_table);
+        let destination_type = types_table
+            .variable_types()
+            .get(&destination_symbol)
+            .unwrap();
+        let destination_stack_variable =
+            self.make_stack_variable(builder, destination_type, destination_stack_offset);
+        let as_rc = Rc::new(RefCell::new(destination_stack_variable));
+        let ir_assign = IrAssign::new(as_rc.clone(), init_operation);
+        // do not need to save variable to builder as a new one is created for each repl function
+        builder
+            .current_block_mut()
+            .add_statement(IrStatement::Assign(ir_assign));
+        as_rc
+    }
 }

dmc-lib/src/diagnostic.rs

@@ -1,3 +1,5 @@
+pub type Diagnostics = Vec<Diagnostic>;
+
 #[derive(Debug)]
 pub struct Diagnostic {
     message: String,

dmc-lib/src/error_codes.rs

@@ -1,5 +1,7 @@
 pub type ErrorCode = usize;
 
+pub const LEXER_ERROR: ErrorCode = 1;
+pub const PARSE_ERROR: ErrorCode = 2;
 pub const SYMBOL_NOT_FOUND: ErrorCode = 13;
 pub const SYMBOL_ALREADY_DECLARED: ErrorCode = 14;
 pub const BINARY_INCOMPATIBLE_TYPES: ErrorCode = 15;

dmc-lib/src/ir/ir_assign.rs

@@ -2,8 +2,9 @@ use crate::constants_table::ConstantsTable;
 use crate::ir::assemble::{Assemble, InstructionsBuilder};
 use crate::ir::ir_operation::IrOperation;
 use crate::ir::ir_variable::{IrVariable, IrVariableDescriptor, IrVrVariableDescriptor};
-use crate::ir::register_allocation::{OffsetCounter, VrUser};
+use crate::ir::register_allocation::VrUser;
 use crate::ir::util::propagate_spills;
+use crate::offset_counter::OffsetCounter;
 use dvm_lib::instruction::Instruction;
 use std::cell::RefCell;
 use std::collections::{HashMap, HashSet};
@@ -57,7 +58,7 @@ impl VrUser for IrAssign {
     fn propagate_stack_offsets(&mut self, counter: &mut OffsetCounter) {
         let mut borrowed_destination = self.destination.borrow_mut();
         if let IrVariableDescriptor::Stack(stack_variable) = borrowed_destination.descriptor_mut() {
-            stack_variable.set_offset(counter.next());
+            stack_variable.use_next_offset(counter);
         }
     }
 }

dmc-lib/src/ir/ir_block.rs

@@ -2,7 +2,8 @@ use crate::constants_table::ConstantsTable;
 use crate::ir::assemble::{Assemble, InstructionsBuilder};
 use crate::ir::ir_statement::IrStatement;
 use crate::ir::ir_variable::IrVrVariableDescriptor;
-use crate::ir::register_allocation::{HasVrUsers, OffsetCounter, VrUser};
+use crate::ir::register_allocation::{HasVrUsers, VrUser};
+use crate::offset_counter::OffsetCounter;
 use std::cell::RefCell;
 use std::collections::{HashMap, HashSet};
 use std::fmt::{Display, Formatter};
@@ -122,6 +123,7 @@ impl Display for IrBlock {
 #[cfg(test)]
 mod tests {
     use crate::diagnostic::Diagnostic;
+    use crate::offset_counter::OffsetCounter;
     use crate::parser::parse_compilation_unit;
     use crate::symbol_table::SymbolTable;
     use crate::types_table::TypesTable;
@@ -155,7 +157,7 @@ mod tests {
             .unwrap();
 
         let mut main_ir = main.to_ir(&symbol_table, &types_table, None);
-        let (register_assignments, _) = main_ir.assign_registers(2);
+        let register_assignments = main_ir.assign_registers(2, &mut OffsetCounter::new());
         assert_eq!(register_assignments.len(), 4);
 
         Ok(())

dmc-lib/src/ir/ir_function.rs

@@ -4,6 +4,7 @@ use crate::ir::ir_block::IrBlock;
 use crate::ir::ir_parameter::IrParameter;
 use crate::ir::ir_variable::IrVrVariableDescriptor;
 use crate::ir::register_allocation::HasVrUsers;
+use crate::offset_counter::OffsetCounter;
 use crate::type_info::TypeInfo;
 use dvm_lib::vm::function::Function;
 use std::cell::RefCell;
@@ -36,11 +37,14 @@ impl IrFunction {
     pub fn assign_registers(
         &mut self,
         register_count: usize,
-    ) -> (HashMap<IrVrVariableDescriptor, usize>, isize) {
-        self.entry.borrow_mut().assign_registers(register_count)
+        offset_counter: &mut OffsetCounter,
+    ) -> HashMap<IrVrVariableDescriptor, usize> {
+        self.entry
+            .borrow_mut()
+            .assign_registers(register_count, offset_counter)
     }
 
-    pub fn assemble(&self, stack_size: isize, constants_table: &mut ConstantsTable) -> Function {
+    pub fn assemble(&self, stack_size: usize, constants_table: &mut ConstantsTable) -> Function {
         let mut builder = InstructionsBuilder::new();
         self.entry.borrow().assemble(&mut builder, constants_table);
         let instructions = builder.take_instructions();

dmc-lib/src/ir/ir_statement.rs

@@ -5,7 +5,8 @@ use crate::ir::ir_call::IrCall;
 use crate::ir::ir_return::IrReturn;
 use crate::ir::ir_set_field::IrSetField;
 use crate::ir::ir_variable::IrVrVariableDescriptor;
-use crate::ir::register_allocation::{OffsetCounter, VrUser};
+use crate::ir::register_allocation::VrUser;
+use crate::offset_counter::OffsetCounter;
 use std::collections::{HashMap, HashSet};
 use std::fmt::{Display, Formatter};
 

dmc-lib/src/ir/ir_variable.rs

@@ -1,3 +1,4 @@
+use crate::offset_counter::OffsetCounter;
 use crate::type_info::TypeInfo;
 use dvm_lib::instruction::Location;
 use std::collections::HashSet;
@@ -20,9 +21,16 @@ impl IrVariable {
         }
     }
 
-    pub fn new_stack(name: Rc<str>, block_id: usize, type_info: TypeInfo) -> Self {
+    pub fn new_stack_with_offset(
+        name: Rc<str>,
+        block_id: usize,
+        type_info: &TypeInfo,
+        offset: isize,
+    ) -> Self {
+        let mut descriptor = IrStackVariableDescriptor::new(name, block_id);
+        descriptor.set_offset(offset);
         Self {
-            descriptor: IrVariableDescriptor::Stack(IrStackVariableDescriptor::new(name, block_id)),
+            descriptor: IrVariableDescriptor::Stack(descriptor),
             type_info: type_info.clone(),
         }
     }
@@ -177,6 +185,12 @@ impl IrStackVariableDescriptor {
         self.name.clone()
     }
 
+    pub fn use_next_offset(&mut self, offset_counter: &mut OffsetCounter) {
+        if self.offset.is_none() {
+            self.offset = Some(offset_counter.next() as isize);
+        }
+    }
+
     pub fn set_offset(&mut self, offset: isize) {
         self.offset = Some(offset);
     }

dmc-lib/src/ir/register_allocation.rs

@@ -1,4 +1,5 @@
 use crate::ir::ir_variable::IrVrVariableDescriptor;
+use crate::offset_counter::OffsetCounter;
 use std::collections::{HashMap, HashSet};
 
 pub type InterferenceGraph = HashMap<IrVrVariableDescriptor, HashSet<IrVrVariableDescriptor>>;
@@ -104,7 +105,8 @@ pub trait HasVrUsers {
     fn assign_registers(
         &mut self,
         register_count: usize,
-    ) -> (HashMap<IrVrVariableDescriptor, usize>, isize) {
+        offset_counter: &mut OffsetCounter,
+    ) -> HashMap<IrVrVariableDescriptor, usize> {
         let mut spills: HashSet<IrVrVariableDescriptor> = HashSet::new();
         loop {
             let mut interference_graph = self.interference_graph();
@@ -123,12 +125,11 @@ pub trait HasVrUsers {
                     vr_user.propagate_register_assignments(&registers);
                 }
                 // also set offsets
-                let mut offset_counter = OffsetCounter::new();
                 for vr_user in self.vr_users_mut() {
-                    vr_user.propagate_stack_offsets(&mut offset_counter);
+                    vr_user.propagate_stack_offsets(offset_counter);
                 }
 
-                return (registers, offset_counter.get_count());
+                return registers;
             }
         }
     }
@@ -157,26 +158,6 @@ pub trait VrUser {
     }
 }
 
-pub struct OffsetCounter {
-    counter: isize,
-}
-
-impl OffsetCounter {
-    pub fn new() -> Self {
-        Self { counter: 0 }
-    }
-
-    pub fn next(&mut self) -> isize {
-        let offset = self.counter;
-        self.counter += 1;
-        offset
-    }
-
-    pub fn get_count(&self) -> isize {
-        self.counter
-    }
-}
-
 #[derive(Debug)]
 struct WorkItem {
     vr: IrVrVariableDescriptor,

dmc-lib/src/lexer.rs

@@ -34,49 +34,40 @@ impl<'a> Lexer<'a> {
             }
         }
 
-        let token = if chunk.starts_with("->") {
-            Token::new(self.position, self.position + 2, TokenKind::RightArrow)
-        } else if chunk.starts_with("-") {
-            Token::new(self.position, self.position + 1, TokenKind::Minus)
-        } else if chunk.starts_with("(") {
-            Token::new(self.position, self.position + 1, TokenKind::LeftParentheses)
-        } else if chunk.starts_with(")") {
-            Token::new(
-                self.position,
-                self.position + 1,
-                TokenKind::RightParentheses,
-            )
-        } else if chunk.starts_with("*") {
-            Token::new(self.position, self.position + 1, TokenKind::Star)
-        } else if chunk.starts_with("/") {
-            Token::new(self.position, self.position + 1, TokenKind::Slash)
-        } else if chunk.starts_with("%") {
-            Token::new(self.position, self.position + 1, TokenKind::Modulo)
-        } else if chunk.starts_with("+") {
-            Token::new(self.position, self.position + 1, TokenKind::Plus)
-        } else if chunk.starts_with("&") {
-            Token::new(self.position, self.position + 1, TokenKind::Ampersand)
-        } else if chunk.starts_with("^") {
-            Token::new(self.position, self.position + 1, TokenKind::Caret)
-        } else if chunk.starts_with("|") {
-            Token::new(self.position, self.position + 1, TokenKind::Bar)
-        } else if chunk.starts_with("=") {
-            Token::new(self.position, self.position + 1, TokenKind::Equals)
-        } else if chunk.starts_with(",") {
-            Token::new(self.position, self.position + 1, TokenKind::Comma)
-        } else if chunk.starts_with(":") {
-            Token::new(self.position, self.position + 1, TokenKind::Colon)
-        } else if chunk.starts_with(".") {
-            Token::new(self.position, self.position + 1, TokenKind::Dot)
-        } else if chunk.starts_with("[") {
-            Token::new(self.position, self.position + 1, TokenKind::LeftSquare)
-        } else if chunk.starts_with("]") {
-            Token::new(self.position, self.position + 1, TokenKind::RightSquare)
-        } else if chunk.starts_with("<") {
-            Token::new(self.position, self.position + 1, TokenKind::Lt)
-        } else if chunk.starts_with(">") {
-            Token::new(self.position, self.position + 1, TokenKind::Gt)
+        let mut chars = chunk.chars();
+        let current = chars.next().unwrap(); // safe because we return None if chunk is empty above
+        let peek = chars.next();
+
+        let (end, kind) = match current {
+            '-' => {
+                if let Some(peek) = peek {
+                    if peek == '>' {
+                        (self.position + 2, TokenKind::RightArrow)
                     } else {
+                        (self.position + 1, TokenKind::Minus)
+                    }
+                } else {
+                    (self.position + 1, TokenKind::Minus)
+                }
+            }
+            '(' => (self.position + 1, TokenKind::LeftParentheses),
+            ')' => (self.position + 1, TokenKind::RightParentheses),
+            '[' => (self.position + 1, TokenKind::LeftSquare),
+            ']' => (self.position + 1, TokenKind::RightSquare),
+            '<' => (self.position + 1, TokenKind::Lt),
+            '>' => (self.position + 1, TokenKind::Gt),
+            '*' => (self.position + 1, TokenKind::Star),
+            '/' => (self.position + 1, TokenKind::Slash),
+            '%' => (self.position + 1, TokenKind::Modulo),
+            '+' => (self.position + 1, TokenKind::Plus),
+            '&' => (self.position + 1, TokenKind::Ampersand),
+            '^' => (self.position + 1, TokenKind::Caret),
+            '|' => (self.position + 1, TokenKind::Bar),
+            '=' => (self.position + 1, TokenKind::Equals),
+            ',' => (self.position + 1, TokenKind::Comma),
+            '.' => (self.position + 1, TokenKind::Dot),
+            ':' => (self.position + 1, TokenKind::Colon),
+            _ => {
                 // more than one char token
                 if chunk.starts_with(|c: char| c.is_ascii_digit()) {
                     // number literal
@@ -105,12 +96,12 @@ impl<'a> Lexer<'a> {
                             }
                         }
                         if found_fraction {
-                        Token::new(self.position, end, TokenKind::DoubleLiteral)
+                            (end, TokenKind::DoubleLiteral)
                         } else {
-                        Token::new(self.position, end, TokenKind::IntegerLiteral)
+                            (end, TokenKind::IntegerLiteral)
                         }
                     } else {
-                    Token::new(self.position, end, TokenKind::IntegerLiteral)
+                        (end, TokenKind::IntegerLiteral)
                     }
                 } else if chunk.starts_with("\"") {
                     // string literal
@@ -127,9 +118,13 @@ impl<'a> Lexer<'a> {
                         }
                     }
                     if !terminated {
-                    return Some(Err(LexerError::new(LexerErrorKind::UnterminatedString)));
+                        return Some(Err(LexerError::new(
+                            self.position,
+                            end,
+                            LexerErrorKind::UnterminatedString,
+                        )));
                     }
-                Token::new(self.position, end, TokenKind::String)
+                    (end, TokenKind::String)
                 } else {
                     // keyword or identifier
                     let mut prefix = String::new();
@@ -142,9 +137,11 @@ impl<'a> Lexer<'a> {
                     }
 
                     if prefix.len() == 0 {
-                    return Some(Err(LexerError::new(LexerErrorKind::UnrecognizedCharacter(
-                        chunk.chars().next().unwrap(),
-                    ))));
+                        return Some(Err(LexerError::new(
+                            self.position,
+                            self.position + 1,
+                            LexerErrorKind::UnrecognizedCharacter(chunk.chars().next().unwrap()),
+                        )));
                     }
 
                     let token_kind = match prefix.as_str() {
@@ -159,9 +156,11 @@ impl<'a> Lexer<'a> {
                         "ctor" => TokenKind::Ctor,
                         _ => TokenKind::Identifier,
                     };
-                Token::new(self.position, self.position + prefix.len(), token_kind)
+                    (self.position + prefix.len(), token_kind)
+                }
             }
         };
+        let token = Token::new(self.position, end, kind);
         self.position += token.end() - token.start();
         Some(Ok(token))
     }
@@ -169,12 +168,22 @@ impl<'a> Lexer<'a> {
 
 #[derive(Debug, Eq, PartialEq)]
 pub struct LexerError {
+    start: usize,
+    end: usize,
     kind: LexerErrorKind,
 }
 
 impl LexerError {
-    pub fn new(kind: LexerErrorKind) -> Self {
-        Self { kind }
+    pub fn new(start: usize, end: usize, kind: LexerErrorKind) -> Self {
+        Self { start, end, kind }
+    }
+
+    pub fn start(&self) -> usize {
+        self.start
+    }
+
+    pub fn end(&self) -> usize {
+        self.end
     }
 
     pub fn kind(&self) -> LexerErrorKind {

dmc-lib/src/lib.rs

@@ -5,6 +5,7 @@ pub mod error_codes;
 pub mod intrinsics;
 pub mod ir;
 pub mod lexer;
+pub mod offset_counter;
 pub mod parser;
 pub mod scope;
 pub mod source_range;

dmc-lib/src/offset_counter.rs

@@ -0,0 +1,23 @@
+pub struct OffsetCounter {
+    counter: usize,
+}
+
+impl OffsetCounter {
+    pub fn new() -> Self {
+        Self { counter: 0 }
+    }
+
+    pub fn with_base(base: usize) -> Self {
+        Self { counter: base }
+    }
+
+    pub fn next(&mut self) -> usize {
+        let offset = self.counter;
+        self.counter += 1;
+        offset
+    }
+
+    pub fn get_count(&self) -> usize {
+        self.counter
+    }
+}

dmc-lib/src/parser.rs

@@ -19,23 +19,34 @@ use crate::ast::parameter::Parameter;
 use crate::ast::statement::Statement;
 use crate::ast::string_literal::StringLiteral;
 use crate::ast::type_use::TypeUse;
-use crate::diagnostic::Diagnostic;
-use crate::lexer::Lexer;
+use crate::diagnostic::{Diagnostic, Diagnostics};
+use crate::error_codes::{LEXER_ERROR, PARSE_ERROR};
+use crate::lexer::{Lexer, LexerErrorKind};
 use crate::source_range::SourceRange;
 use crate::token::{Token, TokenKind};
+use crate::{handle_diagnostics, ok_or_err_diagnostics};
 use std::str::FromStr;
 
-pub fn parse_compilation_unit(input: &str) -> Result<CompilationUnit, Vec<Diagnostic>> {
+pub type ParseResult<T> = Result<T, Diagnostics>;
+
+pub fn parse_compilation_unit(input: &str) -> ParseResult<CompilationUnit> {
     let mut parser = Parser::new(input);
+    parser.advance()?; // get started
     parser.compilation_unit()
 }
 
-pub fn parse_expression(input: &str) -> Result<Expression, Vec<Diagnostic>> {
+pub fn parse_expression(input: &str) -> ParseResult<Expression> {
     let mut parser = Parser::new(input);
-    parser.advance(); // get started
+    parser.advance()?; // get started
     parser.expression()
 }
 
+pub fn parse_let_statement(input: &str) -> ParseResult<LetStatement> {
+    let mut parser = Parser::new(input);
+    parser.advance()?;
+    parser.let_statement()
+}
+
 macro_rules! matches_expression_first {
     ( $token_kind : expr ) => {
         matches!(
@@ -96,96 +107,116 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn advance(&mut self) {
+    fn advance(&mut self) -> Result<(), Diagnostics> {
+        fn fetch(lexer: &mut Lexer) -> Result<Option<Token>, Diagnostics> {
+            let mut diagnostics = vec![];
+            let mut maybe_token: Option<Token> = None;
+            while let Some(lexer_result) = lexer.next() {
+                match lexer_result {
+                    Ok(token) => {
+                        maybe_token = Some(token);
+                        break;
+                    }
+                    Err(lexer_error) => {
+                        let diagnostic = match lexer_error.kind() {
+                            LexerErrorKind::UnterminatedString => Diagnostic::new(
+                                "Unterminated string literal.",
+                                lexer_error.start(),
+                                lexer_error.end(),
+                            )
+                            .with_error_code(LEXER_ERROR),
+                            LexerErrorKind::UnrecognizedCharacter(c) => Diagnostic::new(
+                                &format!("Unrecognized character: {}", c),
+                                lexer_error.start(),
+                                lexer_error.end(),
+                            )
+                            .with_error_code(LEXER_ERROR),
+                        };
+                        diagnostics.push(diagnostic);
+                    }
+                }
+            }
+            ok_or_err_diagnostics!(maybe_token, diagnostics)
+        }
+
         if self.lookahead.is_some() {
             // we've advanced at least once
             self.current = self.lookahead.take();
-            self.lookahead = match self.lexer.next() {
-                None => None,
-                Some(result) => match result {
-                    Ok(token) => Some(token),
-                    Err(lexer_error) => {
-                        panic!("{:?}", lexer_error);
-                    }
-                },
-            }
+            self.lookahead = fetch(&mut self.lexer)?;
+            Ok(())
         } else if self.lookahead.is_none() && self.current.is_some() {
             // we're on the last token
             self.current = None;
+            Ok(())
         } else {
             // we've not yet advanced, so fetch both
-            // current
-            match self.lexer.next() {
-                None => {}
-                Some(result) => match result {
-                    Ok(token) => {
-                        self.current = Some(token);
-                    }
-                    Err(lexer_error) => {
-                        panic!("{:?}", lexer_error);
-                    }
-                },
-            }
-            // lookahead
-            match self.lexer.next() {
-                None => {}
-                Some(result) => match result {
-                    Ok(token) => {
-                        self.lookahead = Some(token);
-                    }
-                    Err(lexer_error) => {
-                        panic!("{:?}", lexer_error);
-                    }
-                },
-            }
+            self.current = fetch(&mut self.lexer)?;
+            self.lookahead = fetch(&mut self.lexer)?;
+            Ok(())
         }
     }
 
-    fn expect_advance(&mut self, token_kind: TokenKind) -> Result<Token, Vec<Diagnostic>> {
-        match self.current.take() {
-            None => Err(vec![
+    fn join_kinds(kinds: &[TokenKind]) -> String {
+        kinds
+            .iter()
+            .map(|kind| format!("{:?}", kind))
+            .collect::<Vec<_>>()
+            .join(", ")
+    }
+
+    fn get_expected_but_found(kinds: &[TokenKind], found: &Token) -> Diagnostic {
         Diagnostic::new(
-                    &format!("Expected {:?} but found end-of-input.", token_kind),
-                    self.input.len(),
-                    self.input.len(),
+            &format!("Unexpected token: {:?}.", found.kind()),
+            found.start(),
+            found.end(),
         )
-                .with_reporter(file!(), line!()),
-            ]),
+        .with_error_code(PARSE_ERROR)
+        .with_primary_label_message(&format!("Expected {}.", Self::join_kinds(kinds)))
+    }
+
+    fn get_expected_but_found_eoi(kinds: &[TokenKind], position: usize) -> Diagnostic {
+        Diagnostic::new("Unexpected end-of-input.", position, position)
+            .with_error_code(PARSE_ERROR)
+            .with_primary_label_message(&format!("Expected {}.", Self::join_kinds(kinds)))
+    }
+
+    fn expect_advance(&mut self, token_kind: TokenKind) -> Result<Token, Diagnostics> {
+        match self.current.take() {
+            None => Err(vec![Self::get_expected_but_found_eoi(
+                &[token_kind],
+                self.input.len(),
+            )]),
             Some(token) => {
                 if token.kind() == token_kind {
-                    self.advance();
+                    self.advance()?;
                     Ok(token)
                 } else {
                     self.advance_until(&[token_kind]);
-                    Err(vec![
-                        Diagnostic::new(
-                            &format!("Expected {:?} but found {:?}", token_kind, token.kind()),
-                            token.start(),
-                            token.end(),
-                        )
-                        .with_reporter(file!(), line!()),
-                    ])
+                    Err(vec![Self::get_expected_but_found(&[token_kind], &token)])
                 }
             }
         }
     }
 
-    fn expect_position_advance(
+    fn expect_immediately_after_advance(
         &mut self,
         token_kind: TokenKind,
-        start_position: usize,
-    ) -> Result<Token, Vec<Diagnostic>> {
+        previous_token: &Token,
+    ) -> Result<Token, Diagnostics> {
         let matched = self.expect_advance(token_kind)?;
-        if matched.start() == start_position {
+        if matched.start() == previous_token.end() {
             Ok(matched)
         } else {
             Err(vec![
                 Diagnostic::new(
-                    &format!("Expected {:?} but found {:?}", token_kind, matched.kind()),
+                    &format!(
+                        "Expected {:?} immediately after previous token.",
+                        token_kind
+                    ),
                     matched.start(),
                     matched.end(),
                 )
-                .with_reporter(file!(), line!()),
+                .with_error_code(PARSE_ERROR),
             ])
         }
     }
@@ -206,13 +237,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn peek_lookahead(&self, token_kind: TokenKind) -> bool {
-        match &self.lookahead {
-            None => panic!("Unexpected end of input."),
-            Some(token) => token.kind() == token_kind,
-        }
-    }
-
     fn sample_input(&self, start: usize, end: usize) -> &'a str {
         &self.input[start..end]
     }
@@ -221,49 +245,40 @@ impl<'a> Parser<'a> {
         self.sample_input(token.start(), token.end())
     }
 
-    pub fn compilation_unit(&mut self) -> Result<CompilationUnit, Vec<Diagnostic>> {
+    fn compilation_unit(&mut self) -> Result<CompilationUnit, Vec<Diagnostic>> {
         let mut functions: Vec<Function> = vec![];
         let mut extern_functions: Vec<ExternFunction> = vec![];
         let mut classes: Vec<Class> = vec![];
 
         let mut diagnostics = vec![];
 
-        self.advance(); // get started
-
         while self.current.is_some() {
             let current = self.get_current();
             match current.kind() {
                 TokenKind::Fn | TokenKind::Extern | TokenKind::Class => {
-                    match self.module_level_declaration(
+                    handle_diagnostics!(
+                        self.module_level_declaration(
                             &mut functions,
                             &mut extern_functions,
-                        &mut classes,
-                    ) {
-                        Ok(_) => {}
-                        Err(mut declaration_diagnostics) => {
-                            diagnostics.append(&mut declaration_diagnostics)
-                        }
-                    }
+                            &mut classes
+                        ),
+                        diagnostics
+                    );
                 }
                 _ => {
-                    diagnostics.push(Diagnostic::new(
-                        &format!(
-                            "Expected any of {:?}; found {:?}",
-                            [TokenKind::Fn, TokenKind::Extern, TokenKind::Class],
-                            current.kind()
-                        ),
-                        current.start(),
-                        current.end(),
+                    diagnostics.push(Self::get_expected_but_found(
+                        &[TokenKind::Fn, TokenKind::Extern, TokenKind::Class],
+                        current,
                     ));
-                    self.advance_until(&[TokenKind::Fn, TokenKind::Extern]);
+                    self.advance_until(&[TokenKind::Fn, TokenKind::Extern, TokenKind::Class]);
                 }
             }
         }
-        if diagnostics.is_empty() {
-            Ok(CompilationUnit::new(functions, extern_functions, classes))
-        } else {
-            Err(diagnostics)
-        }
+
+        ok_or_err_diagnostics!(
+            CompilationUnit::new(functions, extern_functions, classes),
+            diagnostics
+        )
     }
 
     fn module_level_declaration(
@@ -307,7 +322,7 @@ impl<'a> Parser<'a> {
 
     fn function(&mut self) -> Result<Function, Vec<Diagnostic>> {
         let is_public = if self.current.is_some() && self.peek_current(TokenKind::Public) {
-            self.advance(); // pub
+            self.advance()?; // pub
             true
         } else {
             false
@@ -356,18 +371,17 @@ impl<'a> Parser<'a> {
             _ => {}
         }
 
-        if diagnostics.is_empty() {
-            Ok(Function::new(
+        ok_or_err_diagnostics!(
+            Function::new(
                 self.token_text(&identifier_token),
                 SourceRange::new(identifier_token.start(), identifier_token.end()),
                 is_public,
                 parameters,
                 return_type,
                 statements,
-            ))
-        } else {
-            Err(diagnostics)
-        }
+            ),
+            diagnostics
+        )
     }
 
     fn extern_function(&mut self) -> Result<ExternFunction, Vec<Diagnostic>> {
@@ -399,16 +413,15 @@ impl<'a> Parser<'a> {
 
         let return_type = self.return_type()?;
 
-        if diagnostics.is_empty() {
-            Ok(ExternFunction::new(
+        ok_or_err_diagnostics!(
+            ExternFunction::new(
                 self.token_text(&identifier_token),
                 SourceRange::new(identifier_token.start(), identifier_token.end()),
                 maybe_parameters.unwrap(),
                 return_type,
-            ))
-        } else {
-            Err(diagnostics)
-        }
+            ),
+            diagnostics
+        )
     }
 
     fn class(&mut self) -> Result<Class, Vec<Diagnostic>> {
@@ -459,18 +472,17 @@ impl<'a> Parser<'a> {
 
         self.expect_advance(TokenKind::End)?;
 
-        if diagnostics.is_empty() {
-            Ok(Class::new(
+        ok_or_err_diagnostics!(
+            Class::new(
                 self.token_text(&identifier_token),
                 SourceRange::new(identifier_token.start(), identifier_token.end()),
                 generic_parameters,
                 maybe_constructor,
                 fields,
                 functions,
-            ))
-        } else {
-            Err(diagnostics)
-        }
+            ),
+            diagnostics
+        )
     }
 
     fn parameter_list(&mut self) -> Result<Vec<Parameter>, Vec<Diagnostic>> {
@@ -487,14 +499,15 @@ impl<'a> Parser<'a> {
                 }
             }
             if self.current.is_some() && self.peek_current(TokenKind::Comma) {
-                self.advance();
+                match self.advance() {
+                    Ok(_) => {}
+                    Err(mut ds) => {
+                        diagnostics.append(&mut ds);
+                    }
+                };
             }
         }
-        if diagnostics.is_empty() {
-            Ok(parameters)
-        } else {
-            Err(diagnostics)
-        }
+        ok_or_err_diagnostics!(parameters, diagnostics)
     }
 
     fn parameter(&mut self) -> Result<Parameter, Vec<Diagnostic>> {
@@ -518,7 +531,7 @@ impl<'a> Parser<'a> {
             let current = self.get_current();
             return match current.kind() {
                 TokenKind::LeftSquare => {
-                    self.advance(); // [
+                    self.advance()?; // [
                     let inner_type_use = self.type_use()?;
                     self.expect_advance(TokenKind::RightSquare)?;
                     todo!()
@@ -527,7 +540,7 @@ impl<'a> Parser<'a> {
                     let identifier_token = self.expect_advance(TokenKind::Identifier)?;
                     let generic_arguments =
                         if self.current.is_some() && self.peek_current(TokenKind::Lt) {
-                            self.advance(); // <
+                            self.advance()?; // <
                             let generic_arguments = self.generic_arguments_list()?;
                             self.expect_advance(TokenKind::Gt)?; // >
                             generic_arguments
@@ -540,19 +553,14 @@ impl<'a> Parser<'a> {
                         generic_arguments,
                     ))
                 }
-                _ => Err(vec![Diagnostic::new(
-                    &format!(
-                        "Expected LeftSquare or Identifier; found: {:?}",
-                        current.kind()
-                    ),
-                    current.start(),
-                    current.end(),
+                _ => Err(vec![Self::get_expected_but_found(
+                    &[TokenKind::LeftSquare, TokenKind::Identifier],
+                    current,
                 )]),
             };
         }
-        Err(vec![Diagnostic::new(
-            "Expected LeftSquare or Identifier; found end of input.",
-            self.input.len(),
+        Err(vec![Self::get_expected_but_found_eoi(
+            &[TokenKind::LeftSquare, TokenKind::Identifier],
             self.input.len(),
         )])
     }
@@ -562,7 +570,7 @@ impl<'a> Parser<'a> {
         while self.current.is_some() && matches_type_use_first!(self.get_current().kind()) {
             generic_arguments.push(self.type_use()?);
             if self.current.is_some() && self.peek_current(TokenKind::Comma) {
-                self.advance(); // comma
+                self.advance()?; // comma
             } else {
                 break;
             }
@@ -576,7 +584,7 @@ impl<'a> Parser<'a> {
         while self.current.is_some() && self.peek_current(TokenKind::Identifier) {
             parameters.push(self.generic_parameter()?);
             if self.current.is_some() && self.peek_current(TokenKind::Plus) {
-                self.advance(); // +
+                self.advance()?; // +
             } else {
                 break;
             }
@@ -589,11 +597,11 @@ impl<'a> Parser<'a> {
         let identifier = self.expect_advance(TokenKind::Identifier)?;
         let mut extends_list: Vec<TypeUse> = vec![];
         if self.current.is_some() && self.peek_current(TokenKind::Colon) {
-            self.advance(); // :
+            self.advance()?; // :
             while self.current.is_some() && matches_type_use_first!(self.get_current().kind()) {
                 extends_list.push(self.type_use()?);
                 if self.current.is_some() && self.peek_current(TokenKind::Comma) {
-                    self.advance(); // ,
+                    self.advance()?; // ,
                 } else {
                     break;
                 }
@@ -623,34 +631,25 @@ impl<'a> Parser<'a> {
                 }
                 _ => {
                     let lookahead = self.lookahead.as_ref().unwrap();
-                    return Err(vec![Diagnostic::new(
-                        &format!(
-                            "Expected any of {:?}; found {:?}",
-                            [TokenKind::Mut, TokenKind::Identifier, TokenKind::Fn],
-                            lookahead.kind()
-                        ),
-                        lookahead.start(),
-                        lookahead.end(),
+                    return Err(vec![Self::get_expected_but_found(
+                        &[TokenKind::Mut, TokenKind::Identifier, TokenKind::Fn],
+                        lookahead,
                     )]);
                 }
             }
             Ok(())
         } else {
             let current = self.current.as_ref().unwrap();
-            Err(vec![Diagnostic::new(
-                &format!(
-                    "Expected any of {:?}; found end-of-input.",
-                    [TokenKind::Mut, TokenKind::Identifier, TokenKind::Fn]
-                ),
-                current.end(),
-                current.end(),
+            Err(vec![Self::get_expected_but_found(
+                &[TokenKind::Mut, TokenKind::Identifier, TokenKind::Fn],
+                current,
             )])
         }
     }
 
     fn constructor(&mut self) -> Result<Constructor, Vec<Diagnostic>> {
         let is_public = if self.current.is_some() && self.peek_current(TokenKind::Public) {
-            self.advance();
+            self.advance()?;
             true
         } else {
             false
@@ -700,14 +699,14 @@ impl<'a> Parser<'a> {
 
     fn field(&mut self) -> Result<Field, Vec<Diagnostic>> {
         let is_public = if self.current.is_some() && self.peek_current(TokenKind::Public) {
-            self.advance();
+            self.advance()?;
             true
         } else {
             false
         };
 
         let is_mut = if self.current.is_some() && self.peek_current(TokenKind::Mut) {
-            self.advance();
+            self.advance()?;
             true
         } else {
             false
@@ -716,14 +715,14 @@ impl<'a> Parser<'a> {
         let identifier = self.expect_advance(TokenKind::Identifier)?;
 
         let declared_type = if self.current.is_some() && self.peek_current(TokenKind::Colon) {
-            self.advance(); // colon
+            self.advance()?; // colon
             Some(self.type_use()?)
         } else {
             None
         };
 
         let initializer = if self.current.is_some() && self.peek_current(TokenKind::Equals) {
-            self.advance(); // equals
+            self.advance()?; // equals
             Some(self.expression()?)
         } else {
             None
@@ -751,7 +750,7 @@ impl<'a> Parser<'a> {
         self.expect_advance(TokenKind::Let)?;
 
         let is_mut = if self.current.is_some() && self.peek_current(TokenKind::Mut) {
-            self.advance();
+            self.advance()?;
             true
         } else {
             false
@@ -790,7 +789,7 @@ impl<'a> Parser<'a> {
     fn bitwise_or_expression(&mut self) -> Result<Expression, Vec<Diagnostic>> {
         let mut result = self.bitwise_xor_expression()?;
         while self.current.is_some() && self.peek_current(TokenKind::Bar) {
-            self.advance(); // |
+            self.advance()?; // |
             let rhs = self.bitwise_xor_expression()?;
             let source_range =
                 SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -807,7 +806,7 @@ impl<'a> Parser<'a> {
     fn bitwise_xor_expression(&mut self) -> Result<Expression, Vec<Diagnostic>> {
         let mut result = self.bitwise_and_expression()?;
         while self.current.is_some() && self.peek_current(TokenKind::Caret) {
-            self.advance(); // ^
+            self.advance()?; // ^
             let rhs = self.bitwise_and_expression()?;
             let source_range =
                 SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -824,7 +823,7 @@ impl<'a> Parser<'a> {
     fn bitwise_and_expression(&mut self) -> Result<Expression, Vec<Diagnostic>> {
         let mut result = self.shift_expression()?;
         while self.current.is_some() && self.peek_current(TokenKind::Ampersand) {
-            self.advance(); // &
+            self.advance()?; // &
             let rhs = self.shift_expression()?;
             let source_range =
                 SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -844,9 +843,9 @@ impl<'a> Parser<'a> {
             let current = self.get_current();
             match current.kind() {
                 TokenKind::Lt => {
-                    let second_lt_start = current.start() + 1;
-                    self.advance(); // first <
-                    self.expect_position_advance(TokenKind::Lt, second_lt_start)?; // second <
+                    let previous_cloned = current.clone();
+                    self.advance()?; // first <
+                    self.expect_immediately_after_advance(TokenKind::Lt, &previous_cloned)?; // second <
                     let rhs = self.additive_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -858,9 +857,9 @@ impl<'a> Parser<'a> {
                     ));
                 }
                 TokenKind::Gt => {
-                    let second_gt_start = current.start() + 1;
-                    self.advance(); // first >
-                    self.expect_position_advance(TokenKind::Gt, second_gt_start)?; // second gt
+                    let previous_cloned = current.clone();
+                    self.advance()?; // first >
+                    self.expect_immediately_after_advance(TokenKind::Gt, &previous_cloned)?; // second gt
                     let rhs = self.additive_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -883,7 +882,7 @@ impl<'a> Parser<'a> {
             let current = self.get_current();
             match current.kind() {
                 TokenKind::Plus => {
-                    self.advance(); // plus
+                    self.advance()?; // plus
                     let rhs = self.multiplicative_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -895,7 +894,7 @@ impl<'a> Parser<'a> {
                     ));
                 }
                 TokenKind::Minus => {
-                    self.advance(); // minus
+                    self.advance()?; // minus
                     let rhs = self.multiplicative_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -918,7 +917,7 @@ impl<'a> Parser<'a> {
             let current = self.get_current();
             match current.kind() {
                 TokenKind::Star => {
-                    self.advance(); // multiply
+                    self.advance()?; // multiply
                     let rhs = self.prefix_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -930,7 +929,7 @@ impl<'a> Parser<'a> {
                     ));
                 }
                 TokenKind::Slash => {
-                    self.advance(); // slash
+                    self.advance()?; // slash
                     let rhs = self.prefix_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -942,7 +941,7 @@ impl<'a> Parser<'a> {
                     ))
                 }
                 TokenKind::Modulo => {
-                    self.advance(); // modulo
+                    self.advance()?; // modulo
                     let rhs = self.prefix_expression()?;
                     let source_range =
                         SourceRange::new(result.source_range().start(), rhs.source_range().end());
@@ -967,7 +966,7 @@ impl<'a> Parser<'a> {
             match current.kind() {
                 TokenKind::Minus => {
                     operator_tokens.push(current.clone()); // unfortunately necessary
-                    self.advance();
+                    self.advance()?;
                 }
                 _ => break,
             }
@@ -1010,7 +1009,7 @@ impl<'a> Parser<'a> {
             TokenKind::IntegerLiteral => {
                 let raw = self.token_text(&current);
                 let source_range = SourceRange::new(current.start(), current.end());
-                self.advance();
+                self.advance()?;
                 Ok(Expression::Integer(IntegerLiteral::new(
                     i32::from_str(raw).unwrap(),
                     source_range,
@@ -1019,7 +1018,7 @@ impl<'a> Parser<'a> {
             TokenKind::DoubleLiteral => {
                 let raw = self.token_text(&current);
                 let source_range = SourceRange::new(current.start(), current.end());
-                self.advance();
+                self.advance()?;
                 Ok(Expression::Double(DoubleLiteral::new(
                     f64::from_str(raw).unwrap(),
                     source_range,
@@ -1028,7 +1027,7 @@ impl<'a> Parser<'a> {
             TokenKind::String => {
                 let with_quotes = self.token_text(&current);
                 let source_range = SourceRange::new(current.start(), current.end());
-                self.advance();
+                self.advance()?;
                 Ok(Expression::String(StringLiteral::new(
                     &with_quotes[1..with_quotes.len() - 1],
                     source_range,
@@ -1037,7 +1036,7 @@ impl<'a> Parser<'a> {
             TokenKind::Identifier => {
                 let declared_name = self.token_text(&current);
                 let source_range = SourceRange::new(current.start(), current.end());
-                self.advance();
+                self.advance()?;
                 Ok(Expression::Identifier(Identifier::new(
                     declared_name,
                     source_range,
@@ -1065,7 +1064,7 @@ impl<'a> Parser<'a> {
         let mut expressions = vec![];
         expressions.push(self.expression()?);
         while self.current.is_some() && self.peek_current(TokenKind::Comma) {
-            self.advance(); // comma
+            self.advance()?; // comma
             expressions.push(self.expression()?);
         }
         Ok(expressions)

dmc-lib/src/symbol_table/mod.rs

@@ -89,6 +89,10 @@ impl SymbolTable {
         self.current_scope_id.unwrap()
     }
 
+    pub fn change_scope(&mut self, scope_id: usize) {
+        self.current_scope_id = Some(scope_id);
+    }
+
     fn scope(&self, scope_id: usize) -> &Scope {
         &self.scopes[scope_id]
     }

dvm-lib/src/vm/function.rs

@@ -5,7 +5,7 @@ use std::rc::Rc;
 pub struct Function {
     name: Rc<str>,
     parameter_count: usize,
-    stack_size: isize,
+    stack_size: usize,
     instructions: Vec<Instruction>,
 }
 
@@ -13,7 +13,7 @@ impl Function {
     pub fn new(
         name: Rc<str>,
         parameter_count: usize,
-        stack_size: isize,
+        stack_size: usize,
         instructions: Vec<Instruction>,
     ) -> Self {
         Self {
@@ -36,7 +36,7 @@ impl Function {
         self.parameter_count
     }
 
-    pub fn stack_size(&self) -> isize {
+    pub fn stack_size(&self) -> usize {
         self.stack_size
     }
 

dvm-lib/src/vm/mod.rs

@@ -115,15 +115,15 @@ impl<'a> CallFrame<'a> {
         self.ip += 1;
     }
 
-    fn stack(&self) -> &[Operand] {
+    pub fn stack(&self) -> &[Operand] {
         &self.stack
     }
 
-    fn stack_mut(&mut self) -> &mut Vec<Operand> {
+    pub fn stack_mut(&mut self) -> &mut Vec<Operand> {
         &mut self.stack
     }
 
-    fn fp(&self) -> usize {
+    pub fn fp(&self) -> usize {
         self.fp
     }
 
@@ -163,11 +163,11 @@ impl<'a> CallStack<'a> {
         self.call_frames.pop().unwrap()
     }
 
-    fn top(&self) -> &CallFrame<'a> {
+    pub fn top(&self) -> &CallFrame<'a> {
         &self.call_frames[self.call_frames.len() - 1]
     }
 
-    fn top_mut(&mut self) -> &mut CallFrame<'a> {
+    pub fn top_mut(&mut self) -> &mut CallFrame<'a> {
         let call_frames_len = self.call_frames.len();
         &mut self.call_frames[call_frames_len - 1] // dang borrow checker
     }
@@ -190,18 +190,24 @@ impl<'a> CallStack<'a> {
     }
 }
 
-pub fn call<'a>(
-    context: &'a DvmContext,
-    registers: &mut Vec<Operand>,
-    call_stack: &mut CallStack<'a>,
+pub fn call(
+    context: &DvmContext,
     function_name: &str,
     arguments: &[Value],
+    register_count: usize,
 ) -> Option<Value> {
-    // check if DVM_DEBUG is enabled
-    let debug = std::env::var("DVM_DEBUG")
-        .map(|s| s.eq("1"))
-        .unwrap_or(false);
+    let mut call_stack = CallStack::new();
+    let mut registers = vec![Operand::Null; register_count];
+    prepare_for_instruction_loop(context, function_name, &mut call_stack, arguments);
+    loop_instructions(context, &mut registers, &mut call_stack)
+}
 
+pub fn prepare_for_instruction_loop<'a>(
+    context: &'a DvmContext,
+    function_name: &str,
+    call_stack: &mut CallStack<'a>,
+    arguments: &[Value],
+) {
     let function = context
         .functions
         .get(function_name)
@@ -222,12 +228,22 @@ pub fn call<'a>(
     call_stack.top_mut().set_fp(arguments.len());
 
     // ensure enough stack space
-    call_stack.top_mut().stack_mut().resize(
-        arguments.len() + (function.stack_size() as usize),
-        Operand::Null,
-    );
+    call_stack
+        .top_mut()
+        .stack_mut()
+        .resize(arguments.len() + function.stack_size(), Operand::Null);
+}
+
+pub fn loop_instructions<'a>(
+    context: &'a DvmContext,
+    registers: &mut Vec<Operand>,
+    call_stack: &mut CallStack<'a>,
+) -> Option<Value> {
+    // check if DVM_DEBUG is enabled
+    let debug = std::env::var("DVM_DEBUG")
+        .map(|s| s.eq("1"))
+        .unwrap_or(false);
 
-    // container for final return value
     let mut return_value: Option<Value> = None;
 
     while call_stack.maybe_top().is_some()
@@ -296,7 +312,7 @@ pub fn call<'a>(
                 call_stack
                     .top_mut()
                     .stack_mut()
-                    .resize(arg_count + (function.stack_size() as usize), Operand::Null);
+                    .resize(arg_count + function.stack_size(), Operand::Null);
 
                 // set fp for callee frame
                 let callee_frame = call_stack.top_mut();
@@ -690,6 +706,9 @@ pub fn call<'a>(
                     // callee is the bottommost frame ("main" or whatever was called)
                     // set the return value
                     return_value = callee.take_return_value();
+                    // push the frame back on the call stack for further inspection by caller
+                    call_stack.push(callee);
+                    return return_value;
                 }
                 // do not increment ip of the caller; this was done above BEFORE the call
             }

e2e-tests/src/lib.rs

@@ -3,15 +3,15 @@ mod e2e_tests {
     use dmc_lib::constants_table::ConstantsTable;
     use dmc_lib::diagnostic::Diagnostic;
     use dmc_lib::intrinsics::{insert_intrinsic_symbols, insert_intrinsic_types};
+    use dmc_lib::offset_counter::OffsetCounter;
     use dmc_lib::parser::parse_compilation_unit;
     use dmc_lib::symbol_table::SymbolTable;
     use dmc_lib::types_table::TypesTable;
     use dvm_lib::vm::class::Class;
     use dvm_lib::vm::constant::{Constant, StringConstant};
     use dvm_lib::vm::function::Function;
-    use dvm_lib::vm::operand::Operand;
     use dvm_lib::vm::value::Value;
-    use dvm_lib::vm::{CallStack, DvmContext, call};
+    use dvm_lib::vm::{DvmContext, call};
     use std::rc::Rc;
 
     const REGISTER_COUNT: usize = 8;
@@ -56,7 +56,9 @@ mod e2e_tests {
         let mut constants_table = ConstantsTable::new();
 
         for ir_function in &mut ir_functions {
-            let (_, stack_size) = ir_function.assign_registers(REGISTER_COUNT);
+            let mut offset_counter = OffsetCounter::new();
+            ir_function.assign_registers(REGISTER_COUNT, &mut offset_counter);
+            let stack_size = offset_counter.get_count();
             let function = ir_function.assemble(stack_size, &mut constants_table);
             functions.push(function);
         }
@@ -95,16 +97,7 @@ mod e2e_tests {
         function_name: &str,
         arguments: &[Value],
     ) -> Option<Value> {
-        let mut registers: Vec<Operand> = vec![Operand::Null; REGISTER_COUNT];
-        let mut call_stack = CallStack::new();
-
-        call(
-            &dvm_context,
-            &mut registers,
-            &mut call_stack,
-            function_name,
-            &arguments,
-        )
+        call(&dvm_context, function_name, &arguments, REGISTER_COUNT)
     }
 
     fn assert_result(input: &str, function_name: &str, arguments: &[Value], expected_value: Value) {