Add match expressions

crates/ditto-ast/src/expression.rs

@@ -58,6 +58,25 @@ pub enum Expression {
         /// The expression to evaluate otherwise.
         false_clause: Box<Self>,
     },
+    /// A pattern match.
+    ///
+    /// ```ditto
+    /// match some_expr with
+    /// | Pattern -> another_expr
+    /// ```
+    Match {
+        /// The source span for this expression.
+        span: Span,
+
+        /// The type of the expressions in the `arms`.
+        match_type: Type,
+
+        /// Expression to be matched.
+        expression: Box<Self>,
+
+        /// Patterns to be matched against and their corresponding expressions.
+        arms: NonEmpty<(Pattern, Self)>,
+    },
     /// A value constructor local to the current module, e.g. `Just` and `Ok`.
     LocalConstructor {
         /// The source span for this expression.
@@ -193,6 +212,7 @@ impl Expression {
                 }
             }
             Self::If { output_type, .. } => output_type.clone(),
+            Self::Match { match_type, .. } => match_type.clone(),
             Self::LocalConstructor {
                 constructor_type, ..
             } => constructor_type.clone(),
@@ -220,6 +240,7 @@ impl Expression {
             Self::Function { span, .. } => *span,
             Self::Call { span, .. } => *span,
             Self::If { span, .. } => *span,
+            Self::Match { span, .. } => *span,
             Self::LocalConstructor { span, .. } => *span,
             Self::ImportedConstructor { span, .. } => *span,
             Self::LocalVariable { span, .. } => *span,
@@ -296,3 +317,33 @@ impl FunctionBinder {
         }
     }
 }
+
+/// A pattern to be matched.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum Pattern {
+    /// A local constructor pattern.
+    LocalConstructor {
+        /// The source span for this pattern.
+        span: Span,
+        /// `Just`
+        constructor: ProperName,
+        /// Pattern arguments to the constructor.
+        arguments: Vec<Self>,
+    },
+    /// An importedf constructor pattern.
+    ImportedConstructor {
+        /// The source span for this pattern.
+        span: Span,
+        /// `Maybe.Just`
+        constructor: FullyQualifiedProperName,
+        /// Pattern arguments to the constructor.
+        arguments: Vec<Self>,
+    },
+    /// A variable binding pattern.
+    Variable {
+        /// The source span for this pattern.
+        span: Span,
+        /// Name to bind.
+        name: Name,
+    },
+}

crates/ditto-checker/golden-tests/type-errors/match_unification_error_0.ditto

@@ -0,0 +1,8 @@
+module Test exports (..);
+
+type Foo = Foo;
+type Bar = Bar;
+
+nah = 
+  match Foo with
+  | Bar -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_0.error

@@ -0,0 +1,12 @@
+
+  × types don't unify
+   ╭─[golden:5:1]
+ 5 │ 
+ 6 │ nah = 
+ 7 │   match Foo with
+ 8 │   | Bar -> unit;
+   ·     ─┬─
+   ·      ╰── here
+   ╰────
+  help: expected Foo
+        got Bar

crates/ditto-checker/golden-tests/type-errors/match_unification_error_1.ditto

@@ -0,0 +1,10 @@
+module Test exports (..);
+
+type Maybe(a) = Just(a) | Nothing;
+type Foo = Foo;
+type Bar = Bar;
+
+nah = (maybe_foo : Maybe(Foo)) -> 
+  match maybe_foo with
+  | Just(Bar) -> unit
+  | Nothing -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_1.error

@@ -0,0 +1,13 @@
+
+  × types don't unify
+    ╭─[golden:6:1]
+  6 │ 
+  7 │ nah = (maybe_foo : Maybe(Foo)) -> 
+  8 │   match maybe_foo with
+  9 │   | Just(Bar) -> unit
+    ·          ─┬─
+    ·           ╰── here
+ 10 │   | Nothing -> unit;
+    ╰────
+  help: expected Foo
+        got Bar

crates/ditto-checker/golden-tests/type-errors/match_unification_error_2.ditto

@@ -0,0 +1,8 @@
+module Test exports (..);
+
+type Maybe(a) = Just(a) | Nothing;
+
+nah = (maybe_foo : Maybe(Int)) -> 
+  match maybe_foo with
+  | Just(a, b) -> unit
+  | Nothing -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_2.error

@@ -0,0 +1,11 @@
+
+  × wrong number of arguments
+   ╭─[golden:4:1]
+ 4 │ 
+ 5 │ nah = (maybe_foo : Maybe(Int)) -> 
+ 6 │   match maybe_foo with
+ 7 │   | Just(a, b) -> unit
+   ·     ─────┬────
+   ·          ╰── this expects 1 argument
+ 8 │   | Nothing -> unit;
+   ╰────

crates/ditto-checker/golden-tests/type-errors/match_unification_error_3.ditto

@@ -0,0 +1,8 @@
+module Test exports (..);
+
+type Maybe(a) = Just(a) | Nothing;
+
+nah = (maybe_foo : Maybe(Int)) -> 
+  match maybe_foo with
+  | Just -> unit
+  | Nothing -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_3.error

@@ -0,0 +1,11 @@
+
+  × wrong number of arguments
+   ╭─[golden:4:1]
+ 4 │ 
+ 5 │ nah = (maybe_foo : Maybe(Int)) -> 
+ 6 │   match maybe_foo with
+ 7 │   | Just -> unit
+   ·     ──┬─
+   ·       ╰── this expects 1 argument
+ 8 │   | Nothing -> unit;
+   ╰────

crates/ditto-checker/golden-tests/type-errors/match_unification_error_4.ditto

@@ -0,0 +1,8 @@
+module Test exports (..);
+
+type Maybe(a) = Just(a) | Nothing;
+
+nah = (maybe_foo : Maybe(Int)) -> 
+  match maybe_foo with
+  | Just(n) -> unit
+  | Nothing(a) -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_4.error

@@ -0,0 +1,10 @@
+
+  × wrong number of arguments
+   ╭─[golden:5:1]
+ 5 │ nah = (maybe_foo : Maybe(Int)) -> 
+ 6 │   match maybe_foo with
+ 7 │   | Just(n) -> unit
+ 8 │   | Nothing(a) -> unit;
+   ·     ─────┬────
+   ·          ╰── this expects no arguments
+   ╰────

crates/ditto-checker/golden-tests/type-errors/match_unification_error_5.ditto

@@ -0,0 +1,8 @@
+module Test exports (..);
+
+type Maybe(a) = Just(a) | Nothing;
+
+nah = (maybe_foo : Maybe(Int)): Int -> 
+  match maybe_foo with
+  | Just(n) -> unit
+  | Nothing -> unit;

crates/ditto-checker/golden-tests/type-errors/match_unification_error_5.error

@@ -0,0 +1,13 @@
+
+  × types don't unify
+   ╭─[golden:4:1]
+ 4 │ 
+ 5 │ nah = (maybe_foo : Maybe(Int)): Int -> 
+ 6 │   match maybe_foo with
+ 7 │   | Just(n) -> unit
+   ·                ──┬─
+   ·                  ╰── here
+ 8 │   | Nothing -> unit;
+   ╰────
+  help: expected Int
+        got Unit

crates/ditto-checker/src/module/value_declarations/mod.rs

@@ -455,6 +455,18 @@ fn toposort_value_declarations(
                 get_connected_nodes_rec(true_clause, nodes, accum);
                 get_connected_nodes_rec(false_clause, nodes, accum);
             }
+            Expression::Match {
+                expression,
+                head_arm,
+                tail_arms,
+                ..
+            } => {
+                get_connected_nodes_rec(expression, nodes, accum);
+                get_connected_nodes_rec(&head_arm.expression, nodes, accum);
+                for tail_arm in tail_arms.iter() {
+                    get_connected_nodes_rec(&tail_arm.expression, nodes, accum);
+                }
+            }
             Expression::Array(elements) => {
                 if let Some(ref elements) = elements.value {
                     elements.iter().for_each(|element| {

crates/ditto-checker/src/typechecker/env.rs

@@ -1,15 +1,15 @@
 use super::{common::type_variables, Scheme};
 use crate::supply::Supply;
 use ditto_ast::{
-    Expression, FullyQualifiedName, FullyQualifiedProperName, Name, ProperName, QualifiedName,
-    QualifiedProperName, Span, Type,
+    Expression, FullyQualifiedName, FullyQualifiedProperName, Name, Pattern, ProperName,
+    QualifiedName, QualifiedProperName, Span, Type,
 };
 use std::{
     collections::{HashMap, HashSet},
     default::Default,
 };
 
-#[derive(Default)]
+#[derive(Default, Clone)]
 pub struct Env {
     pub constructors: EnvConstructors,
     pub values: EnvValues,
@@ -157,6 +157,26 @@ impl EnvConstructor {
         }
     }
 
+    // REVIEW should this be `into_pattern`?
+    pub fn to_pattern(&self, span: Span, arguments: Vec<Pattern>) -> Pattern {
+        match self {
+            Self::ModuleConstructor { constructor, .. } => Pattern::LocalConstructor {
+                span,
+                constructor: constructor.clone(),
+                arguments,
+            },
+            Self::ImportedConstructor { constructor, .. } => Pattern::ImportedConstructor {
+                span,
+                constructor: constructor.clone(),
+                arguments,
+            },
+        }
+    }
+
+    pub fn get_type(&self, supply: &mut Supply) -> Type {
+        self.get_scheme().instantiate(supply)
+    }
+
     fn get_scheme(&self) -> Scheme {
         match self {
             Self::ModuleConstructor {