this is me following along Robert Nystrom's book "Crafting Interpreters" in C# (tree-walk interpreter implementation).

apart from learning about programming languages in this project I also try to experiment and exercise with C# 15 unions, functional programming elements in C#, and "parse, don't validate" / type-driven design.

diff with the book implementation

• something is probably broken
• explicitly defined AST classes instead of code generation
• no visitor pattern, just pattern matching
• some challenges done in the main branch (ternary operator, string + non-string concatenation, break statements, lambdas, unused variable warnings)
• pattern matching support via is and switch expression
• static checks that can be detected syntactically ('return' outside function, 'this'/'super' outside class) are done in the parser, not resolver, because I like to report them as early as possible

pattern matching

is-expression

expr is pattern, evaluates to bool value

switch-expression

expr switch {
    pattern1: onMatchExpr1,
    pattern2: onMatchExpr2,
}

evaluates to the first matched pattern's onMatchExpr. if no pattern is matched, evaluates to nil.

supported patterns

• literals (expr is "lol", expr is 23, expr is false)
• nil (expr is nil)
• built-in types (expr is str, expr is num, expr is bool)
• declared types, supports class hierarchy (expr is MyClass)
• discard, always matches (expr is _)

example

class Animal {
  init(name) { this.name = name; }
}

class Dog < Animal {
  init(name) { super.init(name); }
}

class Cat < Animal {
  init(name) { super.init(name); }
}

var dog = Dog("Julie");

if (dog is Dog) {
    print "dog is Dog";
}

if (dog.name is "Julie") {
    print "dog is Julie";
}

if (dog is Animal) {
    print "dog is Animal";
}

var kind = dog switch {
    22: "kind of 22",
    "hello": "kind of hello",
    str: "string",
    num: "number",
    Cat: "cat",
    Dog: "dog",
    _: "unknown"
};
print kind; // "dog"

how to build and run

requires .net 11 preview 5 or higher

• dotnet run --project src/Lox -- path/to/script.lox to run a file
• dotnet run --project src/Lox to run REPL