[fine] Type testing with probes and reporting

I'm proud of the test harness here actually. Also fix a bug in
checking!

fine/build.rs

@@ -5,15 +5,17 @@ use std::path::{Path, PathBuf};
 
 fn generate_test_for_file(path: PathBuf) -> String {
     let contents = fs::read_to_string(&path).expect("Unable to read input");
+    let display_path = path.display().to_string();
 
     let mut concrete_stuff: Option<String> = None;
 
     // Start iterating over lines and processing directives....
+    let mut type_assertions = Vec::new();
     let mut lines = contents.lines();
     while let Some(line) = lines.next() {
         let line = match line.strip_prefix("//") {
             Some(line) => line,
-            None => break,
+            None => continue,
         };
 
         let line = line.trim();
@@ -29,11 +31,23 @@ fn generate_test_for_file(path: PathBuf) -> String {
                 concrete.push_str("\n");
             }
             concrete_stuff = Some(concrete);
+        } else if let Some(line) = line.strip_prefix("type:") {
+            let (pos, expected) = line
+                .trim()
+                .split_once(' ')
+                .expect("Mal-formed type expectation");
+            let pos: usize = pos
+                .trim()
+                .parse()
+                .expect(&format!("Unable to parse position '{pos}'"));
+            let expected = expected.trim();
+            type_assertions.push(quote! {
+                crate::assert_type_at(&_tree, &_lines, #pos, #expected, #display_path);
+            });
         }
     }
 
     let concrete_comparison = if let Some(concrete) = concrete_stuff {
-        let display_path = path.display().to_string();
         quote! {
             crate::assert_concrete(&_tree, #concrete, #display_path)
         }
@@ -46,6 +60,7 @@ fn generate_test_for_file(path: PathBuf) -> String {
         fn #name() {
             let (_tree, _lines) = fine::parser::parse(#contents);
             #concrete_comparison;
+            #(#type_assertions)*
         }
     };
 

fine/src/lib.rs

@@ -1,2 +1,3 @@
 pub mod parser;
+pub mod semantics;
 pub mod tokens;

fine/src/parser.rs

@@ -1,6 +1,7 @@
 // NOTE: much of this parser structure derived from
 // https://matklad.github.io/2023/05/21/resilient-ll-parsing-tutorial.html
 use crate::tokens::{Lines, Token, TokenKind, Tokens};
+use std::fmt::Write as _;
 use std::{cell::Cell, num::NonZeroU32};
 
 pub mod old; // Until I decide to delete it.
@@ -18,10 +19,26 @@ impl<'a> SyntaxTree<'a> {
         }
     }
 
-    pub fn add_tree(&mut self, t: Tree<'a>) -> TreeRef {
+    pub fn root(&self) -> Option<TreeRef> {
+        self.root
+    }
+
+    pub fn add_tree(&mut self, mut t: Tree<'a>) -> TreeRef {
         assert!(t.parent.is_none());
         let tr = TreeRef::from_index(self.trees.len());
 
+        t.start_pos = t
+            .children
+            .first()
+            .map(|c| c.start_position(&self))
+            .unwrap_or(0);
+
+        t.end_pos = t
+            .children
+            .last()
+            .map(|c| c.end_position(&self))
+            .unwrap_or(t.start_pos);
+
         // NOTE: Because of the difficulty of holding multiple mutable
         //       references it's this is our best chance to patch up parent
         //       pointers.
@@ -30,14 +47,51 @@ impl<'a> SyntaxTree<'a> {
                 self[*ct].parent = Some(tr);
             }
         }
+
         self.trees.push(t);
         tr
     }
 
-    pub fn dump(&self) -> String {
+    pub fn dump(&self, with_positions: bool) -> String {
-        match self.root {
+        let mut output = String::new();
-            Some(r) => self[r].dump(self),
+        if let Some(r) = self.root {
-            None => String::new(),
+            self[r].dump(self, with_positions, &mut output);
+        }
+        output
+    }
+
+    pub fn start_position(&self, t: TreeRef) -> usize {
+        self[t].start_pos
+    }
+
+    pub fn end_position(&self, t: TreeRef) -> usize {
+        self[t].end_pos
+    }
+
+    pub fn find_tree_at(&self, pos: usize) -> Option<TreeRef> {
+        let mut current = self.root?;
+        let mut tree = &self[current];
+        if pos < tree.start_pos || pos >= tree.end_pos {
+            return None;
+        }
+
+        loop {
+            let mut found = false;
+            for child in &tree.children {
+                if let Child::Tree(next) = child {
+                    let next_tree = &self[*next];
+                    if pos >= next_tree.start_pos && pos < next_tree.end_pos {
+                        found = true;
+                        current = *next;
+                        tree = next_tree;
+                        break;
+                    }
+                }
+            }
+
+            if !found {
+                return Some(current);
+            }
         }
     }
 }
@@ -56,7 +110,7 @@ impl<'a> std::ops::IndexMut<TreeRef> for SyntaxTree<'a> {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq)]
 pub enum TreeKind {
     Error,
     File,
@@ -83,10 +137,34 @@ pub enum TreeKind {
 pub struct Tree<'a> {
     pub kind: TreeKind,
     pub parent: Option<TreeRef>,
+    pub start_pos: usize,
+    pub end_pos: usize,
     pub children: Vec<Child<'a>>,
 }
 
-#[derive(Copy, Clone, Eq, PartialEq)]
+impl<'a> Tree<'a> {
+    pub fn nth_token(&self, index: usize) -> Option<&Token<'a>> {
+        self.children
+            .get(index)
+            .map(|c| match c {
+                Child::Token(t) => Some(t),
+                _ => None,
+            })
+            .flatten()
+    }
+
+    pub fn nth_tree(&self, index: usize) -> Option<TreeRef> {
+        self.children
+            .get(index)
+            .map(|c| match c {
+                Child::Tree(t) => Some(*t),
+                _ => None,
+            })
+            .flatten()
+    }
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
 pub struct TreeRef(NonZeroU32);
 
 impl TreeRef {
@@ -102,13 +180,15 @@ impl TreeRef {
 }
 
 impl<'a> Tree<'a> {
-    pub fn dump(&self, tree: &SyntaxTree<'a>) -> String {
+    pub fn dump(&self, tree: &SyntaxTree<'a>, with_positions: bool, output: &mut String) {
-        let mut output = String::new();
+        let _ = write!(output, "{:?}", self.kind);
-        output.push_str(&format!("{:?}\n", self.kind));
+        if with_positions {
-        for child in self.children.iter() {
+            let _ = write!(output, " [{}, {})", self.start_pos, self.end_pos);
-            child.dump_rec(2, tree, &mut output);
+        }
+        let _ = write!(output, "\n");
+        for child in self.children.iter() {
+            child.dump_rec(2, tree, with_positions, output);
         }
-        output
     }
 }
 
@@ -118,21 +198,52 @@ pub enum Child<'a> {
 }
 
 impl<'a> Child<'a> {
-    fn dump_rec(&self, indent: usize, tree: &SyntaxTree<'a>, output: &mut String) {
+    fn dump_rec(
+        &self,
+        indent: usize,
+        tree: &SyntaxTree<'a>,
+        with_positions: bool,
+        output: &mut String,
+    ) {
         for _ in 0..indent {
-            output.push(' ');
+            let _ = write!(output, " ");
         }
         match self {
-            Child::Token(t) => output.push_str(&format!("{:?}:'{:?}'\n", t.kind, t.as_str())),
+            Child::Token(t) => {
+                let _ = write!(output, "{:?}:'{:?}'", t.kind, t.as_str());
+                if with_positions {
+                    let _ = write!(output, " [{}, {})", t.start, t.start + t.as_str().len());
+                }
+                let _ = write!(output, "\n");
+            }
             Child::Tree(t) => {
                 let t = &tree[*t];
-                output.push_str(&format!("{:?}\n", t.kind));
+                let _ = write!(output, "{:?}", t.kind);
+                if with_positions {
+                    let _ = write!(output, " [{}, {})", t.start_pos, t.end_pos);
+                }
+                let _ = write!(output, "\n");
+
                 for child in t.children.iter() {
-                    child.dump_rec(indent + 2, tree, output);
+                    child.dump_rec(indent + 2, tree, with_positions, output);
                 }
             }
         }
     }
+
+    pub fn start_position(&self, syntax_tree: &SyntaxTree) -> usize {
+        match &self {
+            Child::Token(t) => t.start,
+            Child::Tree(t) => syntax_tree[*t].start_pos,
+        }
+    }
+
+    pub fn end_position(&self, syntax_tree: &SyntaxTree) -> usize {
+        match &self {
+            Child::Token(t) => t.start + t.as_str().len(),
+            Child::Tree(t) => syntax_tree[*t].end_pos,
+        }
+    }
 }
 
 enum ParseEvent<'a> {
@@ -307,6 +418,8 @@ impl<'a> CParser<'a> {
                 ParseEvent::Start { kind } => stack.push(Tree {
                     kind,
                     parent: None,
+                    start_pos: 0,
+                    end_pos: 0,
                     children: Vec::new(),
                 }),
 
@@ -660,6 +773,10 @@ mod tests {
     fn tree_ref_size() {
         // What's the point of doing all that work if the tree ref isn't nice
         // and "small"?
+        //
+        // TODO: This is a dumb optimization because tokens are
+        //       huge so Child is huge no matter what we do. If we retain
+        //       tokens out of line then we can re-visit this optimization.
         assert_eq!(4, std::mem::size_of::<Option<TreeRef>>());
     }
 }

fine/src/semantics.rs

@@ -0,0 +1,510 @@
+use crate::{
+    parser::{Child, SyntaxTree, Tree, TreeKind, TreeRef},
+    tokens::{Lines, TokenKind},
+};
+use std::{cell::RefCell, collections::HashMap, fmt};
+
+// TODO: An error should have:
+//
+//   - a start
+//   - an end
+//   - a focus
+//   - descriptive messages
+//
+// that will have to wait for now
+#[derive(Clone, PartialEq, Eq)]
+pub struct Error {
+    pub start: (usize, usize),
+    pub end: (usize, usize),
+    pub message: String,
+}
+
+impl Error {
+    pub fn new<T>(line: usize, column: usize, message: T) -> Self
+    where
+        T: ToString,
+    {
+        Error {
+            start: (line, column),
+            end: (line, column),
+            message: message.to_string(),
+        }
+    }
+
+    pub fn new_spanned<T>(start: (usize, usize), end: (usize, usize), message: T) -> Self
+    where
+        T: ToString,
+    {
+        Error {
+            start,
+            end,
+            message: message.to_string(),
+        }
+    }
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}:{}: {}", self.start.0, self.start.1, self.message)
+    }
+}
+
+#[derive(Copy, Clone)]
+pub enum Type {
+    // Signals a type error. If you receive this then you know that an error
+    // has already been reported; if you produce this be sure to also note
+    // the error in the errors collection.
+    Error,
+
+    // Signals that the expression has a control-flow side-effect and that no
+    // value will ever result from this expression. Usually this means
+    // everything's fine.
+    Unreachable,
+
+    Nothing,
+    // TODO: Numeric literals should be implicitly convertable, unlike other
+    //       types. Maybe just "numeric literal" type?
+    F64,
+    String,
+    Bool,
+}
+
+impl Type {
+    pub fn is_error(&self) -> bool {
+        match self {
+            Type::Error => true,
+            _ => false,
+        }
+    }
+
+    pub fn compatible_with(&self, other: &Type) -> bool {
+        // TODO: This is wrong; we because of numeric literals etc.
+        match (self, other) {
+            (Type::F64, Type::F64) => true,
+            (Type::String, Type::String) => true,
+            (Type::Bool, Type::Bool) => true,
+            (Type::Unreachable, Type::Unreachable) => true,
+
+            // Avoid introducing more errors
+            (Type::Error, _) => true,
+            (_, Type::Error) => true,
+
+            (_, _) => false,
+        }
+    }
+}
+
+impl fmt::Debug for Type {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
+impl fmt::Display for Type {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use Type::*;
+        match self {
+            Error => write!(f, "<< INTERNAL ERROR >>"),
+            Unreachable => write!(f, "<< UNREACHABLE >>"),
+            Nothing => write!(f, "()"),
+            F64 => write!(f, "f64"),
+            String => write!(f, "string"),
+            Bool => write!(f, "bool"),
+        }
+    }
+}
+
+pub struct Semantics<'a> {
+    // TODO: Do I really want my own copy here? Should we standardize on Arc
+    //       or Rc or some other nice sharing mechanism?
+    syntax_tree: &'a SyntaxTree<'a>,
+    lines: &'a Lines,
+    errors: RefCell<Vec<Error>>,
+    types: RefCell<HashMap<TreeRef, Type>>,
+}
+
+impl<'a> Semantics<'a> {
+    pub fn new(tree: &'a SyntaxTree<'a>, lines: &'a Lines) -> Self {
+        let mut semantics = Semantics {
+            syntax_tree: tree,
+            lines,
+            errors: RefCell::new(vec![]),
+            types: RefCell::new(HashMap::new()),
+        };
+
+        // NOTE: We ensure all the known errors are reported before we move
+        //       on to answering any other questions. We're going to work as
+        //       hard as we can from a partial tree.
+        if let Some(tr) = semantics.syntax_tree.root() {
+            semantics.gather_errors(tr);
+        }
+
+        semantics
+    }
+
+    pub fn snapshot_errors(&self) -> Vec<Error> {
+        (*self.errors.borrow()).clone()
+    }
+
+    fn report_error<T>(&self, position: usize, error: T)
+    where
+        T: ToString,
+    {
+        let (line, col) = self.lines.position(position);
+        self.errors
+            .borrow_mut()
+            .push(Error::new(line, col, error.to_string()));
+    }
+
+    fn report_error_span<T>(&self, start: usize, end: usize, error: T)
+    where
+        T: ToString,
+    {
+        let start = self.lines.position(start);
+        let end = self.lines.position(end);
+        self.errors
+            .borrow_mut()
+            .push(Error::new_spanned(start, end, error.to_string()));
+    }
+
+    fn report_error_tree<T>(&self, tree: &Tree<'a>, error: T)
+    where
+        T: ToString,
+    {
+        self.report_error_span(tree.start_pos, tree.end_pos, error)
+    }
+
+    fn report_error_tree_ref<T>(&self, tree: TreeRef, error: T)
+    where
+        T: ToString,
+    {
+        let tree = &self.syntax_tree[tree];
+        self.report_error_span(tree.start_pos, tree.end_pos, error)
+    }
+
+    fn gather_errors(&mut self, tree: TreeRef) {
+        let mut stack = vec![tree];
+        while let Some(tr) = stack.pop() {
+            let tree = &self.syntax_tree[tr];
+            for child in &tree.children {
+                match child {
+                    Child::Token(t) => {
+                        if t.kind == TokenKind::Error {
+                            self.report_error(t.start, t.as_str());
+                        }
+                    }
+                    Child::Tree(t) => stack.push(*t),
+                }
+            }
+        }
+    }
+
+    pub fn type_of(&self, t: TreeRef, value_required: bool) -> Option<Type> {
+        if let Some(existing) = self.types.borrow().get(&t) {
+            return Some(existing.clone());
+        }
+
+        let tree = &self.syntax_tree[t];
+        let result = match tree.kind {
+            TreeKind::Error => Some(Type::Error),
+            TreeKind::UnaryExpression => self.type_of_unary(tree, value_required),
+            TreeKind::BinaryExpression => self.type_of_binary(tree, value_required),
+            TreeKind::TypeExpression => self.type_of_type_expr(tree, value_required),
+            TreeKind::Block => self.type_of_block(tree, value_required),
+            TreeKind::LiteralExpression => self.type_of_literal(tree),
+            TreeKind::GroupingExpression => self.type_of_grouping(tree, value_required),
+            TreeKind::ConditionalExpression => self.type_of_conditional(tree, value_required),
+            TreeKind::CallExpression => self.type_of_call(tree),
+            TreeKind::Argument => self.type_of_argument(tree),
+
+            TreeKind::LetStatement => Some(Type::Nothing),
+            TreeKind::ReturnStatement => Some(Type::Unreachable),
+            TreeKind::ExpressionStatement => {
+                self.type_of_expression_statement(tree, value_required)
+            }
+            TreeKind::Identifier => self.type_of_identifier(tree),
+            _ => return None,
+        };
+
+        // NOTE: These return `None` if they encounter some problem.
+        let result = result.unwrap_or(Type::Error);
+
+        self.types.borrow_mut().insert(t, result.clone());
+        Some(result)
+    }
+
+    fn type_of_unary(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::UnaryExpression);
+
+        let op = tree.nth_token(0)?;
+        let expr = tree.nth_tree(1)?;
+
+        let argument_type = self
+            .type_of(expr, value_required)
+            .expect("Our argument should be an expression");
+
+        match (op.kind, argument_type) {
+            (TokenKind::Plus, Type::F64) => Some(Type::F64),
+            (TokenKind::Minus, Type::F64) => Some(Type::F64),
+            (TokenKind::Bang, Type::Bool) => Some(Type::Bool),
+
+            // This is dumb and should be punished, probably.
+            (_, Type::Unreachable) => {
+                self.report_error(
+                    op.start,
+                    "cannot apply a unary operator to something that doesn't yield a value",
+                );
+                Some(Type::Error)
+            }
+
+            // Propagate existing errors without additional complaint.
+            (_, Type::Error) => Some(Type::Error),
+
+            (_, arg_type) => {
+                self.report_error(
+                    op.start,
+                    format!(
+                        "cannot apply unary operator '{}' to value of type {}",
+                        op.as_str(),
+                        arg_type
+                    ),
+                );
+                Some(Type::Error)
+            }
+        }
+    }
+
+    fn type_of_binary(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::BinaryExpression);
+        let lhs = self
+            .type_of(tree.nth_tree(0)?, value_required)
+            .expect("must be an expression");
+        let op = tree.nth_token(1)?;
+        let rhs = self
+            .type_of(tree.nth_tree(2)?, value_required)
+            .expect("must be an expression");
+
+        match (op.kind, lhs, rhs) {
+            (
+                TokenKind::Plus | TokenKind::Minus | TokenKind::Star | TokenKind::Slash,
+                Type::F64,
+                Type::F64,
+            ) => Some(Type::F64),
+
+            (TokenKind::Plus, Type::String, Type::String) => Some(Type::String),
+
+            (TokenKind::And | TokenKind::Or, Type::Bool, Type::Bool) => Some(Type::Bool),
+
+            // This is dumb and should be punished, probably.
+            (_, _, Type::Unreachable) => {
+                self.report_error(
+                    op.start,
+                    format!("cannot apply '{op}' to an argument that doesn't yield a value (on the right)"),
+                );
+                Some(Type::Error)
+            }
+            (_, Type::Unreachable, _) => {
+                self.report_error(
+                    op.start,
+                    format!("cannot apply '{op}' to an argument that doesn't yield a value (on the left)"),
+                );
+                Some(Type::Error)
+            }
+
+            // Propagate existing errors without additional complaint.
+            (_, Type::Error, _) => Some(Type::Error),
+            (_, _, Type::Error) => Some(Type::Error),
+
+            // Missed the whole table, it must be an error.
+            (_, left_type, right_type) => {
+                self.report_error(
+                    op.start,
+                    format!("cannot apply binary operator '{op}' to expressions of type '{left_type}' (on the left) and '{right_type}' (on the right)"),
+                );
+                Some(Type::Error)
+            }
+        }
+    }
+
+    fn type_of_type_expr(&self, tree: &Tree, _value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::TypeExpression);
+        Some(Type::Error)
+    }
+
+    fn type_of_block(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::Block);
+
+        if tree.children.len() < 2 {
+            return None;
+        }
+
+        // if tree.children.len() == 2 {
+        //     // Empty blocks generate Nothing.
+        //     return Some(Type::Nothing);
+        // }
+
+        // The type of the block is the type of the last expression.
+        // (But the last child is the closing brace probably?)
+        let last_is_brace = tree.nth_token(tree.children.len() - 1).is_some();
+        let last_index = tree.children.len() - if last_is_brace { 2 } else { 1 };
+
+        let mut is_unreachable = false;
+        for i in 1..last_index {
+            is_unreachable = self
+                .type_of(tree.nth_tree(i)?, false)
+                .map(|t| matches!(t, Type::Unreachable))
+                .unwrap_or(false)
+                || is_unreachable;
+        }
+
+        // NOTE: If for some reason the last statement is unsuitable for a
+        //       type then we consider the type of the block to be Nothing.
+        //       (And explicitly not Error, which is what returning None
+        //       would yield.)
+        let last_type = self
+            .type_of(tree.nth_tree(last_index)?, value_required)
+            .unwrap_or(Type::Nothing);
+
+        // If anything in this block generated an "Unreachable" then the
+        // whole type of the block is "unreachable" no matter what.
+        Some(if is_unreachable {
+            Type::Unreachable
+        } else {
+            last_type
+        })
+    }
+
+    fn type_of_literal(&self, tree: &Tree) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::LiteralExpression);
+
+        let tok = tree.nth_token(0)?;
+        let pig = match tok.kind {
+            TokenKind::Number => Type::F64,
+            TokenKind::String => Type::String,
+            TokenKind::True | TokenKind::False => Type::Bool,
+            _ => panic!("the token {tok} doesn't have a type!"),
+        };
+        Some(pig)
+    }
+
+    fn type_of_grouping(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::GroupingExpression);
+
+        let expr = tree.nth_tree(1)?;
+        Some(
+            self.type_of(expr, value_required)
+                .expect("the thing in the parenthesis must have some type"),
+        )
+    }
+
+    fn type_of_conditional(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::ConditionalExpression);
+
+        let cond_tree = tree.nth_tree(1)?;
+        let cond_type = self.type_of(cond_tree, true).expect("must be expression");
+        let then_type = self
+            .type_of(tree.nth_tree(2)?, value_required)
+            .expect("must be expression");
+
+        let has_else = tree
+            .nth_token(3)
+            .map(|t| t.kind == TokenKind::Else)
+            .unwrap_or(false);
+        let else_type = if has_else {
+            Some(
+                self.type_of(tree.nth_tree(4)?, value_required)
+                    .expect("must be expression"),
+            )
+        } else {
+            None
+        };
+
+        if !cond_type.compatible_with(&Type::Bool) {
+            if !cond_type.is_error() {
+                self.report_error_tree_ref(cond_tree, "conditions must yield a boolean");
+            }
+            Some(Type::Error)
+        } else {
+            match (then_type, else_type) {
+                (Type::Error, _) => Some(Type::Error),
+                (_, Some(Type::Error)) => Some(Type::Error),
+                (_, None) if value_required => {
+                    self.report_error_tree(
+                        tree,
+                        "this conditional expression needs an else arm to produce a value",
+                    );
+                    Some(Type::Error)
+                }
+                (then_type, Some(else_type)) if value_required => {
+                    if else_type.compatible_with(&Type::Unreachable) {
+                        // Doesn't matter if the value is required; the else branch
+                        // will never generate a value for us so let's ignore it.
+                        Some(then_type)
+                    } else if then_type.compatible_with(&Type::Unreachable) {
+                        // Or the then branch is unreachable, same thing with else
+                        // then.
+                        Some(else_type)
+                    } else if !then_type.compatible_with(&else_type) {
+                        self.report_error_tree(
+                            tree,
+                            format!("the type of the `then` branch ({then_type}) must match the type of the `else` branch ({else_type})"),
+                        );
+                        Some(Type::Error)
+                    } else {
+                        Some(then_type)
+                    }
+                }
+                (_, _) => {
+                    assert!(!value_required);
+                    Some(Type::Unreachable)
+                }
+            }
+        }
+    }
+
+    fn type_of_call(&self, tree: &Tree) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::CallExpression);
+        Some(Type::Error)
+    }
+
+    fn type_of_argument(&self, tree: &Tree) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::Argument);
+        Some(Type::Error)
+    }
+
+    fn type_of_expression_statement(&self, tree: &Tree, value_required: bool) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::ExpressionStatement);
+        let last_is_semicolon = tree
+            .nth_token(tree.children.len() - 1)
+            .map(|t| t.kind == TokenKind::Semicolon)
+            .unwrap_or(false);
+
+        let expression_type = self
+            .type_of(tree.nth_tree(0)?, value_required && !last_is_semicolon)
+            .expect("must be expression");
+        Some(match expression_type {
+            Type::Error => Type::Error,
+            Type::Unreachable => Type::Unreachable,
+            _ => {
+                // A semicolon at the end of an expression statement discards
+                // the value, leaving us with nothing.
+                if last_is_semicolon {
+                    Type::Nothing
+                } else {
+                    expression_type
+                }
+            }
+        })
+    }
+
+    fn type_of_identifier(&self, tree: &Tree) -> Option<Type> {
+        assert_eq!(tree.kind, TreeKind::Identifier);
+        todo!()
+    }
+}

fine/src/tokens.rs

@@ -429,7 +429,11 @@ impl<'a> Tokens<'a> {
         };
 
         match c {
-            ' ' | '\t' | '\r' | '\n' => self.whitespace(pos),
+            ' ' | '\t' | '\r' => self.whitespace(pos),
+            '\n' => {
+                self.lines.add_line(pos);
+                self.whitespace(pos)
+            }
             '{' => self.token(pos, TokenKind::LeftBrace),
             '}' => self.token(pos, TokenKind::RightBrace),
             '[' => self.token(pos, TokenKind::LeftBracket),

fine/tests/example_tests.rs

@@ -1,4 +1,6 @@
 use fine::parser::SyntaxTree;
+use fine::semantics::{Semantics, Type};
+use fine::tokens::Lines;
 use pretty_assertions::assert_eq;
 
 fn rebase_concrete(source_path: &str, dump: &str) {
@@ -69,7 +71,7 @@ fn rebase_concrete(source_path: &str, dump: &str) {
 }
 
 fn assert_concrete(tree: &SyntaxTree, expected: &str, source_path: &str) {
-    let dump = tree.dump();
+    let dump = tree.dump(false);
     let rebase = std::env::var("FINE_TEST_REBASE")
         .unwrap_or(String::new())
         .to_lowercase();
@@ -83,4 +85,53 @@ fn assert_concrete(tree: &SyntaxTree, expected: &str, source_path: &str) {
     }
 }
 
+fn assert_type_at(
+    tree: &SyntaxTree,
+    lines: &Lines,
+    pos: usize,
+    expected: &str,
+    _source_path: &str,
+) {
+    let tree_ref = match tree.find_tree_at(pos) {
+        Some(t) => t,
+        None => {
+            println!("Unable to find the subtee at position {pos}! Parsed the tree as:");
+            println!("\n{}", tree.dump(true));
+            panic!("Cannot find tree at position {pos}");
+        }
+    };
+
+    let semantics = Semantics::new(tree, lines);
+    let tree_type = semantics.type_of(tree_ref, true);
+
+    let actual = format!("{}", tree_type.unwrap_or(Type::Error));
+    if actual != expected {
+        println!(
+            "The type of the {:?} tree at position {pos} had the wrong type! Parsed the tree as:",
+            tree[tree_ref].kind
+        );
+        println!("\n{}", tree.dump(true));
+
+        let errors = semantics.snapshot_errors();
+        if errors.len() == 0 {
+            println!("There were no errors reported during type checking.\n");
+        } else {
+            println!(
+                "{} error{} reported during type checking:",
+                errors.len(),
+                if errors.len() == 1 { "" } else { "s" }
+            );
+            for error in &errors {
+                println!("  Error: {error}");
+            }
+            println!();
+        }
+
+        assert_eq!(
+            expected, actual,
+            "The type of the tree at position {pos} was incorrect"
+        );
+    }
+}
+
 include!(concat!(env!("OUT_DIR"), "/generated_tests.rs"));

fine/tests/expression/arithmetic.fine

@@ -20,3 +20,5 @@
 // |     Semicolon:'";"'
 //
 1 * 2 + -3 * 4;
+
+// type: 532 f64

fine/tests/expression/boolean.fine

@@ -20,3 +20,5 @@
 // |     Semicolon:'";"'
 //
 true and false or false and !true;
+
+// type: 549 bool

fine/tests/expression/conditional.fine

@@ -24,3 +24,21 @@
 // |         RightBrace:'"}"'
 //
 if true { "discarded"; 23 } else { 45 }
+
+// Here come some type probes!
+// (type of the condition)
+// type: 667 bool
+//
+// (the discarded expression)
+// type: 674 string
+//
+// (the "then" clause)
+// type: 686 f64
+// type: 689 f64
+//
+// (the "else" clause)
+// type: 696 f64
+// type: 699 f64
+//
+// (the overall expression)
+// type: 664 f64

fine/tests/expression/number.fine

@@ -4,5 +4,7 @@
 // |     LiteralExpression
 // |       Number:'"42"'
 // |     Semicolon:'";"'
-//
+
 42;
+
+// type: 129 f64

fine/tests/expression/strings.fine

@@ -10,3 +10,5 @@
 // |     Semicolon:'";"'
 //
 "Hello " + 'world!';
+
+// type: 261 string