deimos-lang/sketching/november_2025/json.dm

ns jsonexample

fn main()
    let o: obj = match parseJsonObj('{ "a": { "b": { "c": [42] }} }')
        Success(o) => o,
        Failure(err) => panic!("There was an error during Json parsing: ${err}")
    end
    assertEq!(o.a.b.c[0], 42)
end

fn parseJsonObj(s: String) -> Result<obj, JsonParseError>
    let lexer = JsonLexer(s)
    let parser = JsonParser(lexer)
    try
        parser.object()
    catch (parseError: JsonParseError)
        Failure(parseError)
    end
end

enum JsonToken
    LBrace,
    RBrace,
    LSquare,
    RSquare,
    Comma,
    String(String),
    Number(Number),
    Boolean(Boolean)
end

platform class String
    pub fn <R> matchAt(startIndex: Int, matchers: Map<Regex, R | fn (match: String) -> R), opts: obj + { ignoreWhitespace: Boolean }) -> Option<R>
        let slice: String = if opts.ignoreWhitespace then
            /[\t\n\r ]*(<rest>.*)/.match(self[startIndex..]).rest as String
        else
            self[startIndex..]
        end
        for [regex, result] in matchers do
            if regex.test(slice) is Some(match) then
                if result is Fn then
                    Some(result(match))
                else
                    Some(result)
                end
            end
        end
        None
    end
end

platform class Regex

    int RegexMatchBase
        val match: String
        val groups: [String],
        val namedGroups: Map<String, String>
    end

    pub int RegexMatch
        props!::<RegexMatchBase>()
    end

    platform fn doMatch(s) -> Option<RegexMatchBase>

    pub fn match(s: String) -> Option<obj : RegexMatch>
        doMatch(s).map { base ->
            dyn obj {
                ...base,
                propertyMissing: { name: String ->
                    base.namedGroups.get(name).unwrap()
                }
            }
        }
    end

end

class JsonLexer(val source: String)
    var index: Int

    fn matchCurrent() -> Option<JsonToken>
        source.matchAt(index, [
            /\{/ => JsonToken::LBrace,
            /}/ => JsonToken::RBrace,
            /\[/ => JsonToken::LSquare,
            /]/ => JsonToken::RSquare,
            /[0-9]+/ => { rawNumber: String => JsonToken::Number(number::parse(rawNumber))) }
            /true/ => JsonToken::Boolean(true),
            /false/ => JsonToken::False(false),
            // The following is not a good way for matching strings but whatever
            /"%w*"/ => { stringWithQuotes => JsonToken::String(stringWithQuotes[1..-1]) }
        ], obj { ignoreWhitespace: true })
    end

    pub fn nextToken() -> Option<JsonToken>
        matchCurrent().ifSome { token ->
            match token
                JsonToken::LBrace | JsonToken::RBrace | JsonToken::LSquare | JsonToken::RSquare | JsonToken::Comma => index++,
                JsonToken::String(s) => index += s.len() + 2,
                JsonToken::Number(n) => index += n.toString().len(),
                JsonToken::Boolean(b) => match b
                    true => index += 4,
                    false => index += 5,
                end
            end
        }
    end

    pub fn peek() -> Option<JsonToken> alias matchCurrent
end

class JsonParser(val lexer: JsonLexer)

    macro parseError!($message)
        throw JsonParserError($message)
    end

    macro expect!($tokenType)
        match lexer.nextToken()
            Some(token) => match token
                JsonToken::$tokenType => do end,
                _ => parseError!("Expected ${$tokenType}")
            end,
            _ => parseError!("Expected ${$tokenType}")
        end
    end

    macro peek!($tokenType)
        match lexer.peek()
            Some(token) => match token
                JsonToken::$tokenType => true,
                _ => false
            end,
            None => false
        end
    end

    pub fn object() -> obj
        expect!(LBrace)
        let result = obj {}
        while keyValue() is Some((key, value)) do
            result[key] = value
        end
        expect!(RBrace)
        result
    end

    fn keyValue() -> Option<(String, Any)>
        let key = match lexer.peek()
            Some(token) => do
                match token
                    JsonToken::String(s) => s,
                    _ => parseError!("Expected String")
                end
            end,
            _ => do return None end
        end
        let value = value()
        Some((key, value))
    end

    fn value() -> Any
        match lexer.peek()
            Some(token) => match token
                JsonToken::LBrace => object(),
                JsonToken::LSquare => array(),
                _ => do
                    match lexer.nextToken()
                        JsonToken::String(s) => s,
                        JsonToken::Number(n) => n,
                        JsonToken::Boolean(b) => b,
                        _ => parseError!("Expected String, Number, or Boolean")
                    end
                end
            end,
            None => parseError!("Expected LBrace, LSquare, String, Number, or Boolean")
        end
    end

    fn array() -> [Any]
        expect!(LSquare)
        let results: List<Any> = []
        while true do
            match lexer.peek()
                Some(token) => match token
                    JsonToken::LBrace => results << object(),
                    JsonToken::LSquare => results << array(),
                    _ => do
                        match lexer.nextToken()
                            JsonToken::String(s) => results << s,
                            JsonToken::Number(n) => results << n,
                            JsonToken::Boolean(b) => results << b,
                            _ => parseError!("Expected String, Number, or Boolean")
                        end
                    end
                end,
                None => panic! "Expected LBrace, LSquare, String, Number, or Boolean"
            end
            match lexer.nextToken()
                Some(token) => match token
                    JsonToken::Comma => continue,
                    JsonToken::RSquare => break
                end
                _ => panic! "Expected Comma or RSquare"
            end
        end
        expect!(RSquare)
    end

end