resolver: Whoops! Commit early WIP resolver.

libconlang/src/lib.rs

@@ -12,6 +12,8 @@ pub mod parser;
 
 pub mod pretty_printer;
 
+pub mod resolver;
+
 pub mod interpreter;
 
 #[cfg(test)]

libconlang/src/resolver.rs

@@ -0,0 +1,874 @@
+//! Extremely early WIP of a static type-checker/resolver
+//!
+//! This will hopefully become a fully fledged static resolution pass in the future
+use std::collections::HashMap;
+
+#[allow(unused_imports)]
+use crate::ast::preamble::*;
+
+/// Prints like [println] if `debug_assertions` are enabled
+macro debugln($($t:tt)*) {
+    if cfg!(debug_assertions) {
+        println!($($t)*);
+    }
+}
+macro debug($($t:tt)*) {
+    if cfg!(debug_assertions) {
+        print!($($t)*);
+    }
+}
+
+use ty::Type;
+pub mod ty {
+    //! Describes the type of a [Variable](super::Variable)
+    use std::fmt::Display;
+
+    /// Describes the type of a [Variable](super::Variable)
+    #[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+    pub enum Type {
+        #[default]
+        Empty,
+        Int,
+        Bool,
+        Char,
+        String,
+        Float,
+        Fn {
+            args: Vec<Type>,
+            ret: Box<Type>,
+        },
+        Range(Box<Type>),
+        Tuple(Vec<Type>),
+        Never,
+        /// [Inferred](Type::Inferred) is for error messages. DO NOT CONSTRUCT
+        Inferred,
+        Generic(String),
+        /// A function with a single parameter of [Type::ManyInferred]
+        /// is assumed to always be correct.
+        ManyInferred,
+    }
+    impl Type {
+        fn is_empty(&self) -> bool {
+            self == &Type::Empty
+        }
+    }
+    impl Display for Type {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            match self {
+                Type::Empty => "Empty".fmt(f),
+                Type::Int => "integer".fmt(f),
+                Type::Bool => "bool".fmt(f),
+                Type::Char => "char".fmt(f),
+                Type::String => "String".fmt(f),
+                Type::Float => "float".fmt(f),
+                // TODO: clean this up
+                Type::Fn { args, ret } => {
+                    "fn (".fmt(f)?;
+                    let mut args = args.iter();
+                    if let Some(arg) = args.next() {
+                        arg.fmt(f)?;
+                    }
+                    for arg in args {
+                        write!(f, ", {arg}")?
+                    }
+                    ")".fmt(f)?;
+                    if !ret.is_empty() {
+                        write!(f, " -> {ret}")?;
+                    }
+                    Ok(())
+                }
+                Type::Range(t) => write!(f, "{t}..{t}"),
+                Type::Tuple(t) => {
+                    "(".fmt(f)?;
+                    for (idx, ty) in t.iter().enumerate() {
+                        if idx > 0 {
+                            ", ".fmt(f)?;
+                        }
+                        ty.fmt(f)?;
+                    }
+                    ")".fmt(f)
+                }
+                Type::Never => "!".fmt(f),
+                Type::Inferred => "_".fmt(f),
+                Type::Generic(name) => write!(f, "<{name}>"),
+                Type::ManyInferred => "..".fmt(f),
+            }
+        }
+    }
+}
+
+/// Describes the life-cycle of a [Variable]: Whether it's bound, typed, or initialized
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Status {
+    #[default]
+    Bound,
+    Uninitialized(Type),
+    Initialized(Type),
+}
+impl Status {
+    /// Performs type-checking for a [Variable] assignment
+    pub fn assign(&mut self, ty: &Type) -> TyResult<()> {
+        match self {
+            // Variable uninitialized: initialize it
+            Status::Bound => {
+                *self = Status::Initialized(ty.clone());
+                Ok(())
+            }
+            // Typecheck ok! Reuse the allocation for t
+            Status::Uninitialized(t) if t == ty => {
+                *self = Status::Initialized(std::mem::take(t));
+                Ok(())
+            }
+            Status::Initialized(t) if t == ty => Ok(()),
+            // Typecheck not ok.
+            Status::Uninitialized(e) | Status::Initialized(e) => {
+                Err(Error::TypeMismatch { want: ty.clone(), got: e.clone() })
+            }
+        }
+    }
+}
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Variable {
+    /// The unique, global index of this variable
+    pub index: usize,
+    /// The [Status] of this variable
+    pub status: Status,
+    /// The mutability qualifier of this variable
+    pub mutable: bool,
+}
+impl Variable {
+    /// Constructs a new variable with the provided index and mutability
+    pub fn new(index: usize, mutable: bool) -> Self {
+        Self { index, mutable, ..Default::default() }
+    }
+    /// Performs a type-checked assignment on self
+    pub fn assign(&mut self, name: &str, ty: &Type) -> TyResult<()> {
+        let Variable { index, status, mutable } = self;
+        debug!("Typecheck for {name} #{index}: ");
+        let out = match (status, mutable) {
+            // Variable uninitialized: initialize it
+            (Status::Bound, _) => {
+                self.status = Status::Initialized(ty.clone());
+                Ok(())
+            }
+            // Typecheck ok! Reuse the allocation for t
+            (Status::Uninitialized(t), _) if t == ty => {
+                self.status = Status::Initialized(std::mem::take(t));
+                Ok(())
+            }
+            // Reassignment of mutable variable is ok
+            (Status::Initialized(t), true) if t == ty => Ok(()),
+            // Reassignment of immutable variable is not ok
+            (Status::Initialized(_), false) => Err(Error::ImmutableAssign(name.into(), *index)),
+            // Typecheck not ok.
+            (Status::Uninitialized(e) | Status::Initialized(e), _) => {
+                Err(Error::TypeMismatch { want: ty.clone(), got: e.clone() })
+            }
+        };
+        match &out {
+            Ok(_) => debugln!("Ok! ({ty})"),
+            Err(e) => debugln!("Error: {e:?}"),
+        }
+        out
+    }
+    /// Performs the type-checking for a modifying assignment
+    pub fn modify_assign(&self, name: &str, ty: &Type) -> TyResult<()> {
+        let Variable { index, status, mutable } = &self;
+        match (status, mutable) {
+            (Status::Initialized(t), true) if t == ty => Ok(()),
+            (Status::Initialized(t), true) => {
+                Err(Error::TypeMismatch { want: t.clone(), got: ty.clone() })
+            }
+            (Status::Initialized(_), false) => Err(Error::ImmutableAssign(name.into(), *index)),
+            (..) => Err(Error::Uninitialized(name.into(), *index)),
+        }
+    }
+}
+
+/*
+    THE BIG IDEA:
+    - Each `let` statement binds a *different* variable.
+      - Shadowing is a FEATURE
+    - Traversing the tree before program launch allows the Resolver to assign
+      an index to each variable usage in the scope-tree
+    - These indices allow constant-time variable lookup in the interpreter!!!
+    - The added type-checking means fewer type errors!
+
+    REQUIREMENTS FOR FULL TYPE-CHECKING:
+    - Meaningful type expressions in function declarations
+
+    NECESSARY CONSIDERATIONS:
+    - Variable binding happens AFTER the initialization expression is run.
+      - If a variable is *entirely* unbound before being referenced,
+        it'll still error.
+      - This is *intentional*, and ALLOWS shadowing previous variables.
+      - In my experience, this is almost NEVER an error :P
+*/
+
+#[derive(Clone, Debug, Default)]
+pub struct Scope {
+    /// A monotonically increasing counter of variable declarations
+    count: usize,
+    /// A dictionary keeping track of type and lifetime information
+    vars: HashMap<String, Variable>,
+}
+impl Scope {
+    /// Bind a [Variable] in Scope
+    pub fn insert(&mut self, name: &str, index: usize, mutable: bool) {
+        self.count += 1;
+        self.vars
+            .insert(name.to_string(), Variable::new(index, mutable));
+    }
+    /// Returns a reference to a [Variable], if `name` is bound
+    pub fn get(&self, name: &str) -> Option<&Variable> {
+        self.vars.get(name)
+    }
+    /// Returns a mutable reference to a [Variable], if `name` is bound
+    pub fn get_mut(&mut self, name: &str) -> Option<&mut Variable> {
+        self.vars.get_mut(name)
+    }
+}
+/// Implements a dynamically scoped namespace
+#[derive(Clone, Debug, Default)]
+pub struct Module {
+    modules: HashMap<String, Module>,
+    vars: HashMap<String, Variable>,
+}
+impl Module {
+    pub fn insert_var(&mut self, name: &str, index: usize, mutable: bool) -> TyResult<()> {
+        if self
+            .vars
+            .insert(name.into(), Variable::new(index, mutable))
+            .is_some()
+        {
+            Err(Error::NonUniqueInModule(name.into()))?;
+        }
+        Ok(())
+    }
+    pub fn insert_module(&mut self, name: String, module: Module) -> TyResult<()> {
+        if self.modules.insert(name.clone(), module).is_some() {
+            Err(Error::NonUniqueInModule(name + "(module)"))?
+        }
+        Ok(())
+    }
+    /// Returns a reference to a [Variable] in this Module, if `name` is bound
+    pub fn get(&self, name: &str) -> Option<&Variable> {
+        self.vars.get(name)
+    }
+    /// Returns a mutable reference to a [Variable] in this Module, if `name` is bound
+    pub fn get_mut(&mut self, name: &str) -> Option<&mut Variable> {
+        self.vars.get_mut(name)
+    }
+
+    pub fn resolve_get(&self, name: &str, path: &[String]) -> Option<&Variable> {
+        if path.is_empty() {
+            return self.get(name);
+        }
+        let module = self.modules.get(&path[0])?;
+        module
+            .resolve_get(name, &path[1..])
+            .or_else(|| self.get(name))
+    }
+    // Returns a reference to the module at a specified path
+    pub fn resolve(&self, path: &[String]) -> TyResult<&Module> {
+        if path.is_empty() {
+            return Ok(self);
+        }
+        let module = self
+            .modules
+            .get(&path[0])
+            .ok_or_else(|| Error::Unbound(path[0].clone()))?;
+        module.resolve(&path[1..])
+    }
+    /// Returns a mutable reference to a Module if one is bound
+    pub fn resolve_mut(&mut self, path: &[String]) -> TyResult<&mut Module> {
+        if path.is_empty() {
+            return Ok(self);
+        }
+        let module = self
+            .modules
+            .get_mut(&path[0])
+            .ok_or_else(|| Error::Unbound(path[0].clone()))?;
+        module.resolve_mut(&path[1..])
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Resolver {
+    /// A monotonically increasing counter of variable declarations
+    count: usize,
+    /// A stack of nested scopes *inside* a function
+    scopes: Vec<Scope>,
+    /// A stack of nested scopes *outside* a function
+    // TODO: Record the name of the module, and keep a stack of the current module
+    // for name resolution
+    modules: Module,
+    /// Describes the current path
+    module: Vec<String>,
+    /// A stack of types
+    types: Vec<Type>,
+}
+
+impl Default for Resolver {
+    fn default() -> Self {
+        let mut new = Self {
+            count: Default::default(),
+            scopes: Default::default(),
+            modules: Default::default(),
+            module: Default::default(),
+            types: Default::default(),
+        };
+        new.register_builtin("print", &[], &[Type::ManyInferred], Type::Empty)
+            .expect("print should not be bound in new Resolver");
+        new
+    }
+}
+
+impl Resolver {
+    pub fn new() -> Self {
+        Default::default()
+    }
+    /// Register a built-in function into the top-level module
+    pub fn register_builtin(
+        &mut self,
+        name: &str,
+        path: &[String],
+        args: &[Type],
+        ret: Type,
+    ) -> TyResult<()> {
+        let module = self.modules.resolve_mut(path)?;
+        module.vars.insert(
+            name.into(),
+            Variable {
+                index: 0,
+                status: Status::Initialized(Type::Fn { args: args.into(), ret: ret.into() }),
+                mutable: false,
+            },
+        );
+        Ok(())
+    }
+    /// Enters a Module Scope
+    pub fn enter_module(&mut self, name: &str) -> TyResult<()> {
+        let module = self.modules.resolve_mut(&self.module)?;
+        module.insert_module(name.into(), Default::default())?;
+        self.module.push(name.into());
+        Ok(())
+    }
+    /// Exits a Module Scope
+    pub fn exit_module(&mut self) -> Option<String> {
+        // Modules stay registered
+        self.module.pop()
+    }
+    /// Enters a Block Scope
+    pub fn enter_scope(&mut self) {
+        self.scopes.push(Default::default());
+    }
+    /// Exits a Block Scope, returning the value
+    pub fn exit_scope(&mut self) -> Option<usize> {
+        self.scopes.pop().map(|scope| scope.count)
+    }
+    //#[deprecated]
+    pub fn push_ty(&mut self, ty: Type) {
+        debugln!("Pushed {ty}");
+        self.types.push(ty)
+    }
+    //#[deprecated]
+    pub fn pop_ty(&mut self) -> TyResult<Type> {
+        self.types
+            .pop()
+            .ok_or_else(|| panic!("Underflow in resolver type stack"))
+    }
+    /// Resolves a name to a [Variable]
+    pub fn get(&self, name: &str) -> TyResult<&Variable> {
+        if let Some(var) = self.scopes.iter().rev().find_map(|s| s.get(name)) {
+            return Ok(var);
+        }
+        self.modules
+            .resolve_get(name, &self.module)
+            .ok_or_else(|| Error::Unbound(name.into()))
+    }
+    /// Mutably resolves a name to a [Variable]
+    pub fn get_mut(&mut self, name: &str) -> TyResult<&mut Variable> {
+        if let Some(var) = self.scopes.iter_mut().rev().find_map(|s| s.get_mut(name)) {
+            return Ok(var);
+        }
+        self.modules
+            .resolve_mut(&self.module)?
+            .get_mut(name)
+            .ok_or_else(|| Error::Unbound(name.into()))
+    }
+    /// Binds a name in the current lexical scope
+    pub fn insert_scope(&mut self, name: &str, mutable: bool) -> TyResult<usize> {
+        self.count += 1;
+        self.scopes
+            .last_mut()
+            .ok_or_else(|| panic!("Stack underflow in resolver?"))?
+            .insert(name, self.count, mutable);
+        Ok(self.count)
+    }
+    /// Binds a name in the current module
+    pub fn insert_module(&mut self, name: &str, mutable: bool) -> TyResult<usize> {
+        self.count += 1;
+        self.modules
+            .resolve_mut(&self.module)?
+            .insert_var(name, self.count, mutable)?;
+        Ok(self.count)
+    }
+    /// Performs scoped type-checking of variables
+    pub fn assign(&mut self, name: &str, ty: &Type) -> TyResult<()> {
+        self.get_mut(name)?.assign(name, ty)
+    }
+}
+/// Manages a sub-function lexical scope
+/// ```rust,ignore
+/// macro scope(self, inner: {...}) -> Result<_, Error>
+/// ```
+macro scope($self:ident, $inner:tt ) {{
+    $self.enter_scope();
+    let scope = (|| $inner)();
+    $self.exit_scope();
+    scope
+}}
+/// Manages a module scope
+/// ```rust,ignore
+/// macro module(self, name: &str, inner: {...}) -> Result<_, Error>
+/// ```
+macro module($self:ident, $name:tt, $inner:tt) {{
+    $self.enter_module($name)?;
+    #[allow(clippy::redundant_closure_call)]
+    let scope = (|| $inner)(); // This is pretty gross, but hey, try {} syntax is unstable too
+    $self.exit_module();
+    scope
+}}
+
+impl Resolver {
+    pub fn visit_empty(&mut self) -> TyResult<()> {
+        debugln!("Got Empty");
+        self.types.push(Type::Empty);
+        Ok(())
+    }
+}
+
+pub trait Resolve {
+    /// Performs variable resolution on self, and returns the type of self.
+    ///
+    /// For expressions, this is the type of the expression.
+    ///
+    /// For declarations, this is Empty.
+    fn resolve(&mut self, _resolver: &mut Resolver) -> TyResult<Type> {
+        Ok(Type::Empty)
+    }
+}
+
+impl Resolve for Start {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Self(program) = self;
+        program.resolve(resolver)
+    }
+}
+impl Resolve for Program {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Self(module) = self;
+        for decl in module {
+            decl.resolve(resolver)?;
+        }
+        // TODO: record the number of module-level assignments into the AST
+        Ok(Type::Empty)
+    }
+}
+impl Resolve for Stmt {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Stmt::Let(value) => value.resolve(resolver),
+            Stmt::Fn(value) => value.resolve(resolver),
+            Stmt::Expr(value) => value.resolve(resolver),
+        }
+    }
+}
+impl Resolve for Let {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Let { name: Name { symbol: Identifier { name, index }, mutable, ty: _ }, init } = self;
+        debugln!("ty> let {name} ...");
+        if let Some(init) = init {
+            let ty = init.resolve(resolver)?;
+            *index = Some(resolver.insert_scope(name, *mutable)?);
+            resolver.get_mut(name)?.assign(name, &ty)?;
+        } else {
+            resolver.insert_scope(name, *mutable)?;
+        }
+        Ok(Type::Empty)
+    }
+}
+impl Resolve for FnDecl {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let FnDecl { name: Name { symbol: Identifier { name, index }, .. }, args, body } = self;
+        debugln!("> fn {name} ...");
+        // register the name at module scope
+        *index = Some(resolver.insert_module(name, false)?);
+        // create a new lexical scope
+        let scopes = std::mem::take(&mut resolver.scopes);
+        // type-check the function body
+        let out = scope!(resolver, {
+            let mut evaluated_args = vec![];
+            for arg in args {
+                evaluated_args.push(arg.resolve(resolver)?)
+            }
+            // TODO: proper typing for return addresses
+            let fn_decl = Type::Fn { args: evaluated_args.clone(), ret: Box::new(Type::Empty) };
+            resolver.get_mut(name)?.assign(name, &fn_decl)?;
+            // Enter the new module
+            module!(resolver, name, { body.resolve(resolver) })
+        });
+        let _ = std::mem::replace(&mut resolver.scopes, scopes);
+        out
+    }
+}
+impl Resolve for Name {
+    fn resolve(&mut self, _resolver: &mut Resolver) -> TyResult<Type> {
+        Ok(Type::Empty)
+    }
+}
+impl Resolve for Block {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Block { let_count: _, statements, expr } = self;
+        scope!(resolver, {
+            for stmt in statements {
+                stmt.resolve(resolver)?;
+            }
+            expr.resolve(resolver)
+        })
+    }
+}
+impl Resolve for Expr {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Expr(expr) = self;
+        expr.resolve(resolver)
+    }
+}
+
+impl Resolve for Operation {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Operation::Assign(value) => value.resolve(resolver),
+            Operation::Binary(value) => value.resolve(resolver),
+            Operation::Unary(value) => value.resolve(resolver),
+            Operation::Call(value) => value.resolve(resolver),
+        }
+    }
+}
+impl Resolve for Assign {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Assign { target, operator, init } = self;
+        // Evaluate the initializer expression
+        let ty = init.resolve(resolver)?;
+        // Resolve the variable
+        match (operator, resolver.get_mut(&target.name)?) {
+            (
+                operator::Assign::Assign,
+                Variable { status: Status::Initialized(_), mutable: false, index },
+            ) => Err(Error::ImmutableAssign(target.name.clone(), *index)),
+            // TODO: make typing more expressive for modifying assignment
+            (_, variable) => variable
+                .modify_assign(&target.name, &ty)
+                .map(|_| Type::Empty),
+        }
+    }
+}
+
+impl Resolve for Binary {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Binary { first, other } = self;
+
+        let mut first = first.resolve(resolver)?;
+        for (op, other) in other {
+            let other = other.resolve(resolver)?;
+            first = resolver.resolve_binary_operator(first, other, op)?;
+        }
+        Ok(first)
+    }
+}
+impl Resolve for Unary {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Unary { operators, operand } = self;
+        let mut operand = operand.resolve(resolver)?;
+        for op in operators {
+            operand = resolver.resolve_unary_operator(operand, op)?;
+        }
+        Ok(operand)
+    }
+}
+/// Resolve [operator]s
+impl Resolver {
+    fn resolve_binary_operator(
+        &mut self,
+        first: Type,
+        other: Type,
+        op: &operator::Binary,
+    ) -> TyResult<Type> {
+        // TODO: check type compatibility for binary ops
+        // TODO: desugar binary ops into function calls, when member functions are a thing
+        eprintln!("Resolve binary operators {first} {op:?} {other}");
+        if first != other {
+            Err(Error::TypeMismatch { want: first, got: other })
+        } else {
+            Ok(first)
+        }
+    }
+    fn resolve_unary_operator(&mut self, operand: Type, op: &operator::Unary) -> TyResult<Type> {
+        // TODO: Allow more expressive unary operator type conversions
+        todo!("Resolve unary operators {op:?} {operand}")
+    }
+}
+impl Resolve for Call {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Call::FnCall(value) => value.resolve(resolver),
+            Call::Primary(value) => value.resolve(resolver),
+        }
+    }
+}
+impl Resolve for FnCall {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let FnCall { callee, args } = self;
+        let mut callee = callee.resolve(resolver)?;
+        for argset in args {
+            // arguments should always be a tuple here
+            let arguments = argset.resolve(resolver)?;
+            let Type::Tuple(arguments) = arguments else {
+                Err(Error::TypeMismatch {
+                    want: Type::Tuple(vec![Type::ManyInferred]),
+                    got: arguments,
+                })?
+            };
+            // Verify that the callee is a function, and the arguments match.
+            // We need the arguments
+            let Type::Fn { args, ret } = callee else {
+                return Err(Error::TypeMismatch {
+                    want: Type::Fn { args: arguments, ret: Type::Inferred.into() },
+                    got: callee,
+                })?;
+            };
+            for (want, got) in args.iter().zip(&arguments) {
+                // TODO: verify generics
+                if let Type::Generic(_) = want {
+                    continue;
+                }
+                if want != got {
+                    return Err(Error::TypeMismatch {
+                        want: Type::Fn { args: arguments, ret: Type::Inferred.into() },
+                        got: Type::Fn { args, ret },
+                    })?;
+                }
+            }
+            callee = *ret;
+        }
+        Ok(callee)
+    }
+}
+impl Resolve for Primary {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Primary::Identifier(value) => value.resolve(resolver),
+            Primary::Literal(value) => value.resolve(resolver),
+            Primary::Block(value) => value.resolve(resolver),
+            Primary::Group(value) => value.resolve(resolver),
+            Primary::Branch(value) => value.resolve(resolver),
+        }
+    }
+}
+
+impl Resolve for Group {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Group::Tuple(tuple) => tuple.resolve(resolver),
+            Group::Single(expr) => expr.resolve(resolver),
+            Group::Empty => Ok(Type::Empty),
+        }
+    }
+}
+
+impl Resolve for Tuple {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Tuple { elements } = self;
+        let mut types = vec![];
+        for expr in elements.iter_mut() {
+            types.push(expr.resolve(resolver)?);
+        }
+        Ok(Type::Tuple(types))
+    }
+}
+
+impl Resolve for Identifier {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Identifier { name, index: id_index } = self;
+        let Variable { index, status, .. } = resolver.get(name)?;
+        *id_index = Some(*index);
+        let ty = match status {
+            Status::Initialized(t) => t,
+            _ => Err(Error::Uninitialized(name.to_owned(), *index))?,
+        };
+        debugln!("ty> Resolved {} #{index}: {ty}", name);
+        Ok(ty.to_owned())
+    }
+}
+impl Resolve for Literal {
+    fn resolve(&mut self, _resolver: &mut Resolver) -> TyResult<Type> {
+        Ok(match self {
+            Literal::String(_) => Type::String,
+            Literal::Char(_) => Type::Char,
+            Literal::Bool(_) => Type::Bool,
+            Literal::Float(_) => Type::Float,
+            Literal::Int(_) => Type::Int,
+        })
+    }
+}
+
+impl Resolve for Flow {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        // TODO: Finish this
+        match self {
+            Flow::While(value) => value.resolve(resolver),
+            Flow::If(value) => value.resolve(resolver),
+            Flow::For(value) => value.resolve(resolver),
+            Flow::Continue(value) => value.resolve(resolver),
+            Flow::Return(value) => value.resolve(resolver),
+            Flow::Break(value) => value.resolve(resolver),
+        }
+    }
+}
+impl Resolve for While {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        // TODO: Finish this
+        // Visit else first, save that to a break-pattern stack in the Resolver,
+        // and check it inside Break::resolve()
+        let While { cond, body, else_ } = self;
+        cond.resolve(resolver)?; // must be Type::Bool
+        body.resolve(resolver)?; // discard
+        else_.resolve(resolver) // compare with returns inside body
+    }
+}
+impl Resolve for If {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let If { cond, body, else_ } = self;
+        let cond = cond.resolve(resolver)?;
+        if Type::Bool != cond {
+            return Err(Error::TypeMismatch { want: Type::Bool, got: cond });
+        }
+        let body_ty = body.resolve(resolver)?;
+        let else_ty = else_.resolve(resolver)?;
+        if body_ty == else_ty {
+            Ok(body_ty)
+        } else {
+            Err(Error::TypeMismatch { want: body_ty, got: else_ty })
+        }
+    }
+}
+
+impl Resolve for For {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let For { var: Identifier { name, index }, iter, body, else_ } = self;
+        debugln!("> for {name} in ...");
+        // Visit the iter expression and get its type
+        let range = iter.resolve(resolver)?;
+        let ty = match range {
+            Type::Range(t) => t,
+            got => Err(Error::TypeMismatch { want: Type::Range(Type::Inferred.into()), got })?,
+        };
+        let body_ty = scope!(resolver, {
+            // bind the variable in the loop scope
+            *index = Some(resolver.insert_scope(name, false)?);
+            resolver.get_mut(name)?.assign(name, &ty)?;
+            body.resolve(resolver)
+        })?;
+        // visit the else block
+        let else_ty = else_.resolve(resolver)?;
+        if body_ty != else_ty {
+            Err(Error::TypeMismatch { want: body_ty, got: else_ty })
+        } else {
+            Ok(body_ty)
+        }
+    }
+}
+impl Resolve for Else {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        let Else { expr } = self;
+        expr.resolve(resolver)
+    }
+}
+
+impl Resolve for Continue {
+    fn resolve(&mut self, _resolver: &mut Resolver) -> TyResult<Type> {
+        // TODO: Finish control flow
+        Ok(Type::Never)
+    }
+}
+impl Resolve for Break {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        // TODO: Finish control flow
+        let Break { expr } = self;
+        expr.resolve(resolver)
+    }
+}
+impl Resolve for Return {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        // TODO: Finish control flow
+        let Return { expr } = self;
+        expr.resolve(resolver)
+    }
+}
+// heakc yea man, generics
+impl<T: Resolve> Resolve for Option<T> {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        match self {
+            Some(t) => t.resolve(resolver),
+            None => Ok(Type::Empty),
+        }
+    }
+}
+impl<T: Resolve> Resolve for Box<T> {
+    fn resolve(&mut self, resolver: &mut Resolver) -> TyResult<Type> {
+        self.as_mut().resolve(resolver)
+    }
+}
+
+use error::{Error, TyResult};
+pub mod error {
+    use super::Type;
+    use std::fmt::Display;
+
+    pub type TyResult<T> = Result<T, Error>;
+
+    impl std::error::Error for Error {}
+    #[derive(Clone, Debug)]
+    pub enum Error {
+        StackUnderflow,
+        // types
+        TypeMismatch { want: Type, got: Type },
+        // modules
+        NonUniqueInModule(String),
+        // lifetimes
+        Uninitialized(String, usize),
+        ImmutableAssign(String, usize),
+        Unbound(String),
+    }
+    impl Display for Error {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            match self {
+                Error::StackUnderflow => "Stack underflow in Resolver".fmt(f),
+                Error::TypeMismatch { want, got } => {
+                    write!(f, "Type error: {want} != {got}")
+                }
+                Error::ImmutableAssign(name, index) => {
+                    write!(f, "Cannot mutate immutable variable {name}(#{index})")
+                }
+                Error::Uninitialized(name, index) => {
+                    write!(f, "{name}(#{index}) was accessed before initialization")
+                }
+                Error::Unbound(name) => write!(f, "{name} not bound before use."),
+                Error::NonUniqueInModule(name) => {
+                    write!(f, "Name {name} not unique at module scope!")
+                }
+            }
+        }
+    }
+}