diff --git a/sketching/april_2025/closure.dm b/sketching/april_2025/closure.dm
new file mode 100644
index 0000000..595007f
--- /dev/null
+++ b/sketching/april_2025/closure.dm
@@ -0,0 +1,530 @@
+pub enum Closure(P, D = Any, S = Any) -> T {
+    Read(ReadClosure), // reads self (where Self is Closure) and/or environment
+    ReadRef(ReadRefClosure), // reads environment/self, return value tied to closure
+    Mut(MutClosure), // mutates self/environment
+    MutRef(MutRefClosure), // mutates self/environment, return value tied to closure
+    Cons(ConsClosure), // consumes self/environment
+}
+
+class Foo(bar: Bar) {}
+
+class Bar(mut i: Int) {}
+
+fn readClosure(cl: Closure(Int) -> String) -> String {
+    cl(42)
+}
+
+fn useReadClosure() {
+    let s = readClosure { i ->
+        'i: ' + i.toString()
+    }
+    assertEq('i: 42', s)
+}
+
+fn mutClosure(cl: mut Closure(Int) -> String) -> String {
+    cl(42)
+}
+
+fn useMutClosure() {
+    let mut x = 0
+    let cl: mut Closure(Int) -> String = mut { i ->
+        let result = 'Val: ' + (i + x)
+        x++
+        result
+    }
+    let s0 = mutClosure cl
+    assertEq('Val: 42', s0)
+    let s1 = mutClosure cl
+    assertEq('Val: 43', s1)
+}
+
+fn consClosure(cl: cons Closure(Int) -> String) -> String {
+    cl(42)
+    // cl(43) // Error! cl was already consumed
+}
+
+fn useConsClosure() {
+    let foo = Foo(Bar(10))
+    let cl: cons Closure(Int) -> String = cons { i ->
+        let fooBarI = foo.bar.i
+        drop(foo)
+        'Val: ' + (i + fooBarI)
+    }
+    let s = consClosure(cl)
+    // let err0 = consClosure(cl) // Error! cl may have been consumed
+    // let err1 = foo // Error! foo may have been consumed
+    assertEq('Val: 52', s)
+}
+
+fn readRefClosure(cl: Closure(foo: Foo) -> Bar ref foo) -> Int {
+    let foo = Foo(Bar(42))
+    let bar = cl(foo)
+    bar.i
+}
+
+fn useReadRefClosure() {
+    let i = readRefClosure { foo -> foo.bar }
+    assertEq(42, i)
+}
+
+fn mutRefClosure(cl: mut Closure(foo: Foo) -> Bar ref foo) -> Int {
+    let foo = Foo(Bar(42))
+    let bar = cl(foo)
+    bar.i
+}
+
+fn useMutRefClosure() {
+    let i = mutRefClosure mut { foo ->
+        foo.bar.i = 84
+        foo.bar
+    }
+    assertEq(84, i)
+}
+
+fn readClosureWithDelegate(cl: Closure(P = Int, D = Foo) -> String) -> String {
+    cl(42)
+}
+
+fn useReadClosureWithDelegate() {
+    let foo = Foo(Bar(42))
+    let cl = { i: Int ->
+        'Val: ' + (i + bar.i) // bar found in delegate
+    }
+    cl.delegate = &foo
+    let s = readClosureWithDelegate(cl)
+    assertEq('Val: 84', s)
+}
+
+class Baz(foo: Foo) {}
+
+fn mutRefClosureWithDelegate(baz: &mut Baz, cl: Closure(Int, baz: &mut Baz)(D = Foo) -> &Bar ref baz) -> String {
+    let foo = Foo(Bar(1))
+    cl.delegate = &foo
+    let bar = cl(2, baz)
+    'Val: ' + bar.i
+}
+
+fn useMutRefClosureWithDelegate() {
+    let cl: Closure(Int, baz: &mut Baz)(D = Foo) -> &Bar ref baz = mut { i, baz ->
+        let iFromDelegate = bar.i
+        baz.foo.bar.i = iFromDelegate + i
+        &baz.foo.bar
+    }
+    let baz = Baz(Foo(Bar(20)))
+    let bar = mutRefClosureWithDelegate(cl)
+    assertEq(3, bar.i)
+}
+
+fn mutClosureWithDelegate(cl: Closure(P = (Int, Int), D = mut Foo) -> String {
+    let foo = Foo(Bar(42))
+    cl.delegate = &mut foo
+    cl(1, 2)
+}
+
+fn useMutClosureWithDelegate() {
+    let cl: Closure(P = (Int, Int), D = mut Foo) -> String = mut { x, y ->
+        bar.i = 3
+        'Val: ' + (x + y + bar.i)
+    }
+    let s = mutClosureWithDelegate(cl)
+    assertEq('Val: 6', s)
+}
+
+pub int ClosureBase {
+    params: Array<ClosureParam>
+    mut resolutionStrategy: ResolutionStrategy
+    ref fn getDelegate() -> Option<&Any>
+    ref fn getSelfObject() -> Option<&Any>
+}
+
+pub enum ResolutionStrategy {
+    DelegateFirst,
+    DelegateOnly,
+    SelfFirst,
+    SelfOnly,
+    None
+}
+
+pub int ConsClosure<T> : ClosureBase {
+    mut fn setDelegate(d: Any) -> Void
+    mut fn setSelf(s: Any) -> Void
+    mut fn call(...args: Array<Any>) -> T
+    def mut op () (...args: Array<Any) -> T = call(...args)
+}
+
+pub int MutClosure<T> : ClosureBase {
+    mut fn setDelegate(d: &mut Any) -> Void
+    mut fn setSelf(s: &mut Any) -> Void
+    mut fn call(...args: Array<Any>) -> T
+    def mut op () (...args: Array<Any>) -> T = call(...args)
+}
+
+pub int MutRefClosure<T> : ClosureBase {
+    mut fn setDelegate(d: &mut Any) -> Void
+    mut fn setSelf(s: &mut Any) -> Void
+    mut ref fn call(...args: Array<Any>) -> T // T ref self
+    def mut ref op () (...args: Array<Any>) -> T = call(...args)
+}
+
+pub int ReadClosure<T> : ClosureBase {
+    mut fn setDelegate(d: &Any) -> Void
+    mut fn setSelf(s: &Any) -> Void
+    fn call(...args: Array<Any>) -> T
+    def op () (...args: Array<Any>) -> T = call(...args)
+}
+
+pub int ReadRefClosure<T> : ClosureBase {
+    mut fn setDelegate(d: &Any) -> Void
+    mut fn setSelf(s: &Any) -> Void
+    ref fn call(...args: Array<Any>) -> T
+    def op () (...args) -> T = call(..args)
+}
+
+pub int ClosureParam {
+    clazz: Class<Any>
+    def op typeof () -> Class<Any> = clazz
+}
+
+// Example of how closures are compiled
+
+fn main() {
+    let is = [1, 2, 3]
+    let cl0 = { acc: String, i: Int ->
+        acc + i
+    }
+    println is.reduce(cl0)
+
+    let x = 10
+    let cl1 = /* read */ { acc: String, i: Int ->
+        acc + (i + x)
+    }
+    println is.reduce(cl1)
+
+    let mut counter = 0
+    let cl2 = mut { acc: String, i: Int ->
+        let result = acc + (i + counter)
+        counter++
+        result
+    }
+    println is.reduce(cl2)
+
+    let mut foo = Foo(Bar(1))
+    let cl3 = cons { acc: String, i: Int ->
+        // do something pointless just for illustration
+        if foo.bar.y % 2 == 0 {
+            foo.bar = Bar(1)
+        }
+        let result = acc + (i + foo.bar.y)
+        foo.bar.y++
+        result
+    }
+    // foo.bar = Bar(42) // Error! foo moved into consuming cl3
+    println [1, 2, 3, 4].reduce(cl3) // "11243645"
+}
+
+// Compiled
+
+@Synthetic
+fn closure_main_cl0(acc: String, i: Int) = acc + i
+
+@Synthetic
+class Closure_main_cl1_env(x: &Int) -> String {}
+
+@Synthetic
+class Closure_main_cl1(env: Closure_main_cl1_env) : ReadClosure<String> {
+    impl fn call(...args: Array<Any>) -> String {
+        let acc: String = args[0]
+        let i: Int = args[1]
+        let x: Int = *env.x // because env.getX() -> &&Int
+        acc + (i + x)
+    }
+}
+
+@Synthetic
+class Closure_main_cl2_env(counter: &mut Int) {}
+
+@Synthetic
+class Closure_main_cl2(env: Closure_main_cl2_env) : MutClosure<String> {
+    impl mut fn call(...args: Array<Any>) -> String {
+        let acc: String = args[0]
+        let i: Int = args[1]
+        let counter = &mut env.counter
+        let result = acc + (i + counter)
+        *counter++
+        result
+    }
+}
+
+@Synthetic
+class Closure_main_cl3(mut foo: Foo) : ConsClosure<String> {
+    impl mut fn call(...args: Array<Any>) -> String {
+        let acc: String = args[0]
+        let i: Int = args[1]
+        if foo.bar.y % 3 == 0 {
+            foo.bar = Bar(1)
+        }
+        let result = acc + (i + foo.bar.y)
+        foo.bar.y++
+        result
+    }
+}
+
+fn main() {
+    let is: List<Int> = ArrayList() {
+        add 1
+        add 2
+        add 3
+    }
+    let cl0 = &closure_main_cl0
+    println is.reduce(cl0)
+
+    let x = 10
+    let cl1_env = HashMap() {
+        put 'x', &x
+    }
+    let cl1: Closure<String> = Closure::ReadClosure(Closure_main_cl1(cl1_env))
+    println is.reduce(cl1)
+
+    let mut counter = 0
+    let cl2_env = Closure_main_cl2_env(&counter)
+    let cl2: Closure<String> = Closure::MutClosure(Closure_main_cl2(cl2_env))
+    println is.reduce(cl2)
+
+    let mut foo = Foo(Bar(1))
+    let cl3: Closure<String> = Closure::ConsClosure(Closure_main_cl3(foo)) // foo moved into closure
+    let tmp0: List<Int> = ArrayList() {
+        addAll 1, 2, 3, 4
+    }
+    println tmp0.reduce(cl3)
+}
+
+// Example
+
+// original
+class Foo {
+    mut fld current = 0
+
+    ref fn getClosure(base: Int) -> (mut Closure(Int) -> Int) {
+        return mut { given ->
+            let result = base + given + current
+            current++
+            result
+        }
+    }
+}
+
+fn main() {
+    let foo = Foo()
+    let cl = foo.getClosure(10)
+    let nums = [cl(1), cl(2), cl(3), cl(4)]
+    assertEq([11, 13, 15, 17], nums)
+}
+
+// Compiles to
+
+@Synthetic
+class Closure_Foo_getClosure_0(base: &Int, current: &mut Int) : MutClosure(Int) -> Int {
+    impl mut fn call(given: Int) -> Int {
+        let result = *base + given + *current
+        *current++
+        result
+    }
+}
+
+class Foo {
+    mut fld current = 0
+
+    ref fn getClosure(base: Int) -> (mut Closure(Int) -> Int) {
+        return Closure_Foo_getClosure_0(&base, &mut current)
+    }
+}
+
+fn main() {
+    /* ... */
+}
+
+// Library code
+
+pub int ReadClosure(P : () = ())<T, D = Any, S = Any> : Into[Self(P)<T, D, S>, Option<D>] {
+    mut fn setDelegate(d: D) -> Void
+    ref fn getDelegate() -> Option<&D>
+    mut fn setSelfObject(s: &S) -> Void
+    ref fn getSelfObject() -> Option<&S>
+    fn call(...params: P) -> T
+    def op () (...params: P) -> T = call(...params)
+
+    def cons fn intoDelegate() -> D {
+        Into[Self(P)<T, D, S>, Option<D>].into()
+    }
+}
+
+pub abstract class AbstractReadClosure(P : () = ())<T, D = Any, S = Any> : ReadClosure(P)<T, D, S> {
+    mut fld delegate: Option<D>
+    mut fld selfObject: Option<&S>
+
+    impl mut fn setDelegate(d: D) {
+        delegate = Some(d)
+    }
+
+    impl ref fn getDelegate() -> Option<&D> {
+        &delegate is Some(d) ? Some(d) : None
+    }
+
+    impl mut fn setSelfObject(s: &S) {
+        selfObject = Some(s)
+    }
+
+    impl ref fn getSelfObject() -> Option<&S> {
+        &selfObject is Some(s) ? Some(s) : None
+    }
+
+    impl Into[Self(P)<T, D, S>, D] {
+        cons fn into() -> Option<D> = delegate
+    }
+}
+
+// User code demonstration
+
+class Foo(bar: Bar) {}
+
+class Bar(num: Int) {}
+
+class Baz(foo: Foo) {
+    pub ref fn getClosure() -> Closure<Int, D = Foo> {
+        return { foo.bar.num + bar.num }
+    }
+}
+
+fn main() {
+    let baz0 = Baz(Foo(Bar(1)))
+    let cl = baz0.getClosure()
+    let delegate = Foo(Bar(2))
+    cl.delegate = delegate
+    let result0 = cl()
+    assertEq(3, result0)
+    let baz1 = Baz(Foo(Bar(2)))
+    cl.selfObject = &baz1
+    let result1 = cl()
+    assertEq(4, result1)
+    let movedDelegate = cl.intoDelegate()
+    assertEq(2, movedDelegate.bar.num)
+}
+
+// User code compiles to
+
+class Foo(bar: Bar) {}
+
+class Bar(num: Int) {}
+
+@Synthetic
+class Closure_Baz_getClosure_0: AbstractReadClosure()<Int, D = Foo> {
+    ctor(selfObject: &Baz) ref selfObject {
+        self.selfObject = Some(selfObject)
+    }
+
+    fn call() -> Int {
+        let t0 = selfObject.unwrap().foo.bar.num
+        let t1 = self.delegate.unwrap().bar.num
+        t0 + t1
+    }
+}
+
+class Baz(foo: Foo) {
+    pub ref fn getClosure() -> Closure<Int, D = Foo> {
+        return Closure_Baz_getClosure_0(&self)
+    }
+}
+
+fn main() {
+    // ...
+}
+
+// fn vs Closure syntax
+
+int FnOrClosureTaker {
+    fn read(f: fn (Int) -> Int) -> Int
+    fn mut(f: mut fn (Int) -> Int) -> Int
+    fn cons(f: cons fn (Int) -> Int) -> Int
+}
+
+class FnOrClosureTakerImpl : FnOrClosureTaker {
+    impl fn read(f: fn (Int) -> Int) {
+        let i0 = f(0)
+        let i1 = f(1)
+        let i2 = f(2)
+        i0 + i1 + i2
+    }
+
+    impl fn mut(f: mut fn (Int) -> Int) {
+        /* same as read */
+    }
+
+    impl fn cons(f: cons fn (Int) -> Int) {
+        let i0 = f()
+        // let i1 = f() // Error! f already consumed
+        i0
+    }
+}
+
+mod User {
+    fn getReader() -> fn (Int) -> Int {
+        return { it * 2 }
+    }
+
+    fn useReader() {
+        let reader = getReader()
+        let fnOrClosureTaker: FnOrClosureTaker = FnOrClosureTakerImpl()
+        let result = fnOrClosureTaker.read(reader)
+        assertEq(6, result)
+    }
+}
+
+// compiles to
+
+mod User {
+    fn getReader_closure_0(it: Int) -> Int {
+        it * 2
+    }
+
+    fn getReader() -> &fn (Int) -> Int {
+        &getReader_closure_0
+    }
+
+    // etc.
+}
+
+// New interfaces?
+
+pub int ReadClosure(P : () = ())<D> -> T : fn (P) -> T {
+    ref fn getDelegate() -> Option<&D>
+    mut fn setDelegate(d: D) -> Void
+    fn call(...p: P) -> T
+    def op () alias call
+}
+
+pub int Iterator<T> {
+    mut fn next() -> Option<T>
+}
+
+pub int Iterable<T> : Into[Self<T>, Iterator<T>] {
+    ref fn iterator() -> Iterator<&T>
+
+    def cons fn intoIterator() = Into[Self, Iterator].into(self)
+}
+
+pub class MyIterable : Iterable<Int> {
+    fld items: List<Int> = [1, 2, 3]
+
+    impl ref fn iterator() -> Iterator<&Int> {
+        let mut index = 0
+        return mut {
+            if index >= items.length {
+                None
+            } else {
+                let next = items[index]
+                index++
+                Some(next)
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/sketching/april_2025/hkt.dm b/sketching/april_2025/hkt.dm
new file mode 100644
index 0000000..c4d2e20
--- /dev/null
+++ b/sketching/april_2025/hkt.dm
@@ -0,0 +1,260 @@
+pub hkt Functor[Self<T>] {
+    fn <U> map(f: fn &T -> U) -> Self<U> // &self implied
+}
+
+pub hkt Applicative[Self<T>] : Functor[Self<T>] {
+    static fn lift(t: T) -> Self<T> // static: no implicit self parameter
+    fn <U, V> apply(us: &Self<U>) -> Self<V> where T : fn (u: &U) -> V
+}
+
+pub hkt Monad[Self<T>] : Applicative[Self<T>] {
+    fn <U> flatMap(f: fn (t: &T) -> Self<U>) -> Self<U>
+}
+
+// Here, T is a separate type parameter from the container's inner type
+// The container's inner type is ..., which can be anything, of any number of parameters
+pub hkt Into[Self<...>, T] {
+    cons fn into() -> T // consumes self
+}
+
+pub hkt Zero[Self<T>] {
+    static fn zero() -> Self<T>
+}
+
+pub int Iterable<T> : Into[Self<T>, Iterator<T>] {
+    /**
+     * Iterator for references to contained item(s).
+     */
+    ref fn iter() -> Iterator<&T> // ref indicates return value cannot outlive self
+
+    def cons fn intoIter() = Into[Self<T>, Iterator<T>].into()
+}
+
+pub type Iterator<T> = () -> Option<T>
+
+pub int Exception {
+    message: Option<String>
+}
+
+pub enum Option<T> : Monad[Self<T>] {
+    Some(T),
+    None;
+
+    class EmptyOptionException(message: Option<String> = Empty) : Exception
+
+    // cons implies that self is consumed (like Rust `self: Self`)
+    cons fn unwrap() -> T = self is Some(t) ? t : throw EmptyOptionException()
+
+    cons fn expect(message: String) -> T {
+        if let Some(t) = self {
+            t
+        } else {
+            throw EmptyOptionException(message)
+        }
+    }
+
+    cons fn unwrapOr(self, other: T) -> T = self is Some(t) ? t : other
+
+    cons fn unwrapOrElse(self, f: fn () -> T) = self is Some(t) ? t : f()
+
+    impl Monad[Self<T>] {
+        fn <U> map(f: fn (t: &T) -> U) -> Self<U> = self is Some(t) ? Some(f(t)) : Empty
+
+        static fn lift(t: T) -> Self<T> = Some(t)
+
+        fn <U, V> apply(us: &Self<U>) -> Self<V> where T : fn (u: U) -> V =
+                self is Some(f) && us is Some(u) ? Some(f(u)) : Empty
+                // note in above that `f` is `&fn (u: U) -> V`, but `f()` is sugar for `(*f)()`
+
+        fn <U> flatMap(f: fn (t: &T) -> Self<U>) -> Self<U> = self is Some(t) ? f(t) : Empty
+    }
+}
+
+pub class OptionT[M]<T>(fld o: M<Option<T>>) : Monad[Self[M]<T>] where M : Monad[Option<T>] {
+    pub fn intoInner() -> M<Option<T>> = Into[Self[M]<T>, M<Option<T>>].into(self)
+
+    pub fn <U> flatMapInner(f: fn (t: &T) -> M<Option<U>>) -> Self[M]<U> {
+        OptionT(
+            Monad[M].flatMap(o) { ts: &Option<T> ->
+                ts is Some(t) ? f(t) : None
+            }
+        )
+    }
+
+    impl Monad[Self[M]<T>] {
+        fn <U> map(f: fn (t: &T) -> U) -> Self[M]<U> {
+            OptionT(
+                    Monad[M].map(o) {
+                        // implicit `it` parameter is `&Option<T>`
+                        // When destructured, `t` is `&T`
+                        it is Some(t) ? Some(f(t)) : None
+                    }
+            )
+        }
+
+        static fn lift(t: T) -> Self[M]<T> = OptionT(Monad[M].lift(Some(t)))
+
+        fn <U, V> apply(us: &Self[M]<U>) -> Self[M]<V> where T : fn (u: &U) -> V {
+            OptionT(
+                    Monad[M].flatMap(o) { fOpt: &Option<fn (u: &U) -> V> ->
+                        fOpt is Some(f)
+                                ? Monad[M].map(us.o) { uOpt: &Option<U> ->
+                                    uOpt is Some(u) ? Some(f(u)) : None
+                                }
+                                : Monad[M].lift(None)
+                    }
+            )
+        }
+
+        fn <U> flatMap(f: fn (t: &T) -> Self[M]<U>) -> Self[M]<U> {
+            OptionT(
+                Monad[M].flatMap(o) {
+                    it is Some(t) ? f(t).o : Monad[M].lift(None)
+                }
+            )
+        }
+    }
+
+    impl Into<Self[M]<T>, M<Option<T>>> {
+        cons fn into() -> M<Option<T>> = o
+    }
+}
+
+pub int List<T> : Iterable<T> + Zero[Self<T>] + Monad[Self<T>] {
+    mut fn add(t: T) -> Void // mut implies &mut self
+
+    // def = default
+    // mut = &mut self
+    // op = operator overload
+    // << = left shift
+    def mut op << (t: T) -> Void = add(t)
+
+    ref fn get(i: Int) -> Option<&T> // ref implies that the returned object cannot outlive self
+                                     // implied &self
+
+    // Alternative syntax for get(i) definition:
+    // fn get(i: Int) -> Option<&T> ref self
+
+    def ref op [] (i: Int) -> Option<&T> = get(i)
+
+    impl Monad[Self<T>] {
+        fn <U> map(f: fn (t: &T) -> U) -> Self<U> {
+            let out: List<U> = Zero[Self<U>].zero()
+            for item in self { // item is &T
+                out << f(item)
+            }
+            out
+        }
+
+        static fn lift(t: T) -> Self<T> = ArrayList(t)
+
+        fn <U ,V> apply(us: &Self<U>) -> Self<V> where T : fn (u: &U) -> V {
+            let out: List<V> = Zero[Self<V>].zero()
+            for f in self { // f is &fn (u: &U) -> V
+                for u in us { // u is &&u
+                    out << f(*u) // syntax sugar for out.add((*f)(*u))
+                }
+            }
+            out
+        }
+
+        fn <U> flatMap(f: fn (t: &T) -> Self<U>) -> Self<U> {
+            let out: List<U> = Zero[Self<U>].zero()
+            for t in self { // t is &T
+                for u in Into[Self<U>, Iterator<U>].into(f(t)) { // u is U
+                    out << u // out take ownership of u
+                }
+            }
+            out
+        }
+    }
+}
+
+pub class LinkedList<T> : List<T> {
+
+    // inner classes are static by default
+    class Node<T>(data: T) {
+        mut next: Option<Self<T>>
+    }
+
+    mut fld head: Option<Node<T>>
+
+    impl mut fn add(t: T) -> Void {
+        match head {
+            None => {
+                head = Some(Node(t))
+            }
+            Some => {
+                let mut current: &Option<Node<T>> = &head
+                while let Some(next) = &current?.next { // get a reference to current.next, or None if either are none
+                    current = next
+                }
+                current.next = Some(Node(t))
+            }
+        }
+    }
+
+    impl ref fn get(index: Int) -> Option<&T> {
+        let mut current: &Option<Node<T>> = &head
+        let mut currentIndex = 0
+        while currentIndex < index {
+            if let Some(next) = &current?.next {
+                currentIndex++
+                current = next
+            } else {
+                break
+            }
+        }
+        current is Some(node) ? Some(&node.data) : None
+    }
+
+    impl ref fn iter() -> Iterator<&T> {
+        let mut current: &Option<Node<T>> = &head
+        () -> {
+            if let Some(node) = current {
+                let data: &T = &node.data
+                current = &node.next
+                Some(data)
+            } else {
+                None
+            }
+        }
+    }
+
+    impl Into[Self<T>, Iterator<T>] {
+        cons fn into() -> Iterator<T> {
+            let mut current: Option<Node<T>> = take(head) // take replaces head with None
+            return () -> {
+                let result = current
+                current = current?.next
+                result is Some(node) ? Some(node.data) : None
+            }
+        }
+    }
+
+    impl Zero[Self<T>] {
+        static fn zero() -> Self<T> = LinkedList()
+    }
+}
+
+fn optionTListDemo() {
+    let oddsOnly: List<Option<Int>> = [
+        Some(1),
+        None,
+        Some(3),
+        None,
+        Some(5)
+    ]
+
+    let liftedOddsOnly: OptionT[]<> = OptionT(oddsOnly)
+
+    let mapped = liftedOddsOnly
+            .flatMapInner { [Some(it), Some(it + 1)] }
+            .map { "Value: ${it.toString()}" }
+
+    for o in mapped.intoInner() {
+        if o is Some(s) {
+            println s
+        }
+    }
+}
diff --git a/sketching/april_2025/string.dm b/sketching/april_2025/string.dm
new file mode 100644
index 0000000..9a8fab1
--- /dev/null
+++ b/sketching/april_2025/string.dm
@@ -0,0 +1,125 @@
+pub int Copy {
+    fn copy() -> Self
+}
+
+pub int Display {
+    fn toString() -> String
+}
+
+pub int LazyDisplay {
+    ref fn toLazyString() -> DString
+}
+
+decl pub class String : Copy, Display {
+    decl impl fn toString() -> String
+    decl impl fn copy() -> Self
+}
+
+pub int DString : Display {
+    parts: Array<DStringPart>
+}
+
+pub enum DStringPart {
+    Constant(String),
+    Display(Display),
+    ReadClosure(ReadClosure<Display>),
+    ReadRefClosure(ReadRefClosure<Display>)
+}
+
+#internal
+class DStringImpl(parts) : DString {
+    mut fld cachedEval: Option<String>
+    mut fld canCache: Option<Boolean>
+
+    fld cachedReads: Map<&ReadClosure ref self, String> = HashMap()
+
+    fn calcCanCache() -> Boolean {
+        for part in parts {
+            if part is ReadRefClosure {
+                return false
+            }
+        }
+        true
+    }
+
+    impl Display {
+        fn toString() -> String {
+            if cachedEval is Some(s) {
+                return s
+            }
+            if canCache is None {
+                canCache = Some(calcCanCache())
+            }
+            let canCache = canCache.unwrap()
+            if canCache {
+                cachedEval = Some(parts.reduce('') { acc, part ->
+                    acc + match part {
+                        Constant(s) => s,
+                        Display(d) => d,
+                        ReadClosure(c) => c(),
+                        ReadRefClosure(c) => throw DumbProgrammerException(
+                                'Cannot cache when parts contains a ReadRefClosure'
+                        )
+                    }
+                })
+                return cachedEval.unwrap()
+            } else {
+                return parts.reduce('') { acc, part ->
+                    acc + match part {
+                        Constant(s) => s,
+                        Display(d) => d,
+                        ReadClosure(c) => {
+                            if let Some(s) = cachedReads.get(&c) {
+                                s
+                            } else {
+                                let s = c()
+                                cachedReads.put(&c, s)
+                                s
+                            }
+                        },
+                        ReadRefClosure(c) => c()
+                    }
+                }
+            }
+        }
+    }
+}
+
+class Foo(mut bar: Bar) {}
+
+class Bar(mut y: Int) {}
+
+fn stringDemo() {
+    let plain: String = 'Hello, World!'
+    let x = 42
+    let withDisplay: String = "Hello! x is $x".
+    let withReadClosure: String = "Hello! x is ${ x + 42 }".
+    let mut foo = Foo(Bar(16))
+    let withReadRefClosure: String = "Hello! y is ${ foo.bar.y }" // "Hello!, y is 16"
+    let lazyDString = `Lazy: y is ${ -> foo.bar.y }` // type is DString, not auto String
+    let eval0: String = lazyDString.toString() // Lazy: y is 16
+    foo.bar.y = 64
+    let eval1: String = lazyDString.toString() // Lazy: y is 64
+}
+
+fn badDStringClosures() {
+    let x = 16
+    let c0 = {
+        x // Int implements copy, so nothing consumed
+    } // ReadClosure
+    let foo = Foo(Bar(8))
+    let c1 = {
+        foo.bar.y
+    } // Read closure
+    let c2_e0 = {
+        foo.bar
+    } // Error! foo.bar cannot be moved out
+    let c2_e1 = {
+        &foo.bar
+    } // Error! reference to foo.bar moved out; closure must declared ref
+    let c2 = {
+        &foo.bar
+    } ref foo // OK, captures foo and return value is tied to lifetime of foo
+    drop(foo)
+    c2() // Error! c2 outlives foo
+}