[fine] Implement the wildcard pattern

fine/src/compiler.rs

@@ -629,6 +629,18 @@ fn compile_is_expression(c: &mut Compiler, tree: &Tree) -> Option<()> {
 fn compile_pattern(c: &mut Compiler, t: TreeRef) -> Option<()> {
     let tree = &c.syntax[t];
 
+    // Let's *try* to generate good code in the presence of a wildcard pattern....
+    let is_wildcard = tree
+        .child_tree_of_kind(c.syntax, TreeKind::WildcardPattern)
+        .is_some();
+
+    let type_expr = tree.child_tree_of_kind(c.syntax, TreeKind::TypeExpression);
+
+    let and_index = tree.children.iter().position(|c| match c {
+        Child::Token(t) => t.kind == TokenKind::And,
+        _ => false,
+    });
+
     // If you have a binding, dup and store now, it is in scope.
     if let Some(binding) = tree.child_tree_of_kind(c.syntax, TreeKind::VariableBinding) {
         if let Some(variable) = binding.nth_token(0) {
@@ -644,31 +656,51 @@ fn compile_pattern(c: &mut Compiler, t: TreeRef) -> Option<()> {
                 ice!(c, t, "is cannot make a non-local, non-variable declaration")
             };
 
-            c.push(Instruction::Dup);
+            // If we aren't a wildcard or we have an attached predicate then
+            // we will need the value on the stack, otherwise we can discard
+            // it.
+            if and_index.is_some() || !is_wildcard {
+                c.push(Instruction::Dup);
+            }
             c.push(Instruction::StoreLocal(*index));
         }
     }
 
-    let type_expr = tree.child_tree_of_kind(c.syntax, TreeKind::TypeExpression)?;
-    compile_type_expr_eq(c, type_expr.nth_tree(0)?);
-
-    let and_index = tree.children.iter().position(|c| match c {
-        Child::Token(t) => t.kind == TokenKind::And,
-        _ => false,
-    });
+    if !is_wildcard {
+        let type_expr = type_expr?;
+        compile_type_expr_eq(c, type_expr.nth_tree(0)?);
+    }
 
     if let Some(and_index) = and_index {
-        // This is the tail end of an "AND" expression.
-        let jump_true_index = c.push(Instruction::JumpTrue(0));
+        let jump_end_index = if is_wildcard {
+            // If the pattern was a wildcard then don't bother with this jump
+            // nonsense; we know the pattern matched, all we need to do is
+            // evaluate the predicate.
+            None
+        } else {
+            // Otherwise test the pattern to see if it passed; if it did then
+            // we need to run the predicate. (This is the back half of an AND
+            // expression.)
+            let jump_true_index = c.push(Instruction::JumpTrue(0));
 
-        c.push(Instruction::PushFalse);
-        let jump_end_index = c.push(Instruction::Jump(0));
+            c.push(Instruction::PushFalse);
+            let jump_end_index = c.push(Instruction::Jump(0));
 
-        c.patch(jump_true_index, |i| Instruction::JumpTrue(i));
+            c.patch(jump_true_index, |i| Instruction::JumpTrue(i));
+            Some(jump_end_index)
+        };
 
         compile_expression(c, tree.nth_tree(and_index + 1)?);
-        c.patch(jump_end_index, |i| Instruction::Jump(i));
-    };
+
+        // If we wound up with a jump what needs patching, patch it.
+        if let Some(jump_end_index) = jump_end_index {
+            c.patch(jump_end_index, |i| Instruction::Jump(i));
+        }
+    } else if is_wildcard {
+        // If there was no predicate *and* the pattern was a wildcard then
+        // I'll just need to push true here.
+        c.push(Instruction::PushTrue);
+    }
 
     OK
 }

fine/src/parser.rs

@@ -163,6 +163,7 @@ pub enum TreeKind {
     MatchExpression,
     MatchBody,
     MatchArm,
+    WildcardPattern,
 }
 
 pub struct Tree<'a> {
@@ -998,7 +999,11 @@ fn pattern(p: &mut CParser, right_power: u8) {
         variable_binding(p);
     }
 
-    type_expr(p);
+    if p.peek() == TokenKind::Underscore {
+        wildcard_pattern(p);
+    } else {
+        type_expr(p);
+    }
 
     if p.eat(TokenKind::And) {
         expression_with_power(p, right_power);
@@ -1016,6 +1021,12 @@ fn variable_binding(p: &mut CParser) {
     p.end(m, TreeKind::VariableBinding);
 }
 
+fn wildcard_pattern(p: &mut CParser) {
+    let m = p.start();
+    p.expect_start(TokenKind::Underscore);
+    p.end(m, TreeKind::WildcardPattern);
+}
+
 fn argument_list(p: &mut CParser) {
     let m = p.start();
 

fine/src/semantics.rs

@@ -857,11 +857,21 @@ impl<'a> Semantics<'a> {
             // parse error, don't make more trouble.
             return Environment::error();
         };
-        self.environment_of_pattern(parent, pattern)
+
+        // The left hand side of the `is` expression is used for wildcard types.
+        let Some(lhs) = tree.nth_tree(0) else {
+            return Environment::error();
+        };
+        self.environment_of_pattern(parent, pattern, lhs)
     }
 
     // NOTE: THIS IS CALLED DIRECTLY, NOT VIA `environment_of` TO AVOID CYCLES.
-    fn environment_of_pattern(&self, parent: EnvironmentRef, tree: &Tree) -> EnvironmentRef {
+    fn environment_of_pattern(
+        &self,
+        parent: EnvironmentRef,
+        tree: &Tree,
+        value_expr: TreeRef,
+    ) -> EnvironmentRef {
         assert_eq!(tree.kind, TreeKind::Pattern);
 
         let Some(binding) = tree.child_tree_of_kind(self.syntax_tree, TreeKind::VariableBinding)
@@ -872,15 +882,30 @@ impl<'a> Semantics<'a> {
         let Some(variable) = binding.nth_token(0) else {
             return Environment::error();
         };
-        let Some(type_expr) = tree.child_of_kind(self.syntax_tree, TreeKind::TypeExpression) else {
-            return Environment::error();
+
+        let is_wildcard = tree
+            .child_of_kind(self.syntax_tree, TreeKind::WildcardPattern)
+            .is_some();
+
+        let variable_decl = if is_wildcard {
+            // If the variable is bound to a wildcard then treat the value
+            // expression as the declaration for the purpose of determining
+            // type.
+            value_expr
+        } else {
+            // Otherwise the binding is to the type expression which must
+            // match for the variable to have a value.
+            let Some(type_expr) = tree.child_of_kind(self.syntax_tree, TreeKind::TypeExpression)
+            else {
+                return Environment::error();
+            };
+            type_expr
         };
 
         // TODO: This binding should be un-assignable! Don't assign to this!
-        //       (UNLESS VERY SPECIFIC CIRCUMSTANCES; WHICH ARE COMPLEX)
 
         let mut env = Environment::new(Some(parent), Location::Local);
-        env.insert(variable, type_expr);
+        env.insert(variable, variable_decl);
         EnvironmentRef::new(env)
     }
 
@@ -1927,6 +1952,7 @@ pub fn check(s: &Semantics) {
             TreeKind::IsExpression => check_is_expression(s, tree),
             TreeKind::VariableBinding => check_variable_binding(s, tree),
             TreeKind::Pattern => check_pattern(s, tree),
+            TreeKind::WildcardPattern => {}
 
             TreeKind::MatchArm => {}
             TreeKind::MatchBody => {}

fine/src/tokens.rs

@@ -57,6 +57,7 @@ pub enum TokenKind {
     Select,
     Selff,
     True,
+    Underscore,
     While,
     Yield,
 }
@@ -351,6 +352,11 @@ impl<'a> Tokens<'a> {
                     return TokenKind::Yield;
                 }
             }
+            '_' => {
+                if ident == "_" {
+                    return TokenKind::Underscore;
+                }
+            }
             _ => (),
         }
 
@@ -605,10 +611,11 @@ mod tests {
 
     test_tokens!(
         more_more_keywords,
-        "in is match",
+        "in is match _",
         (0, In, "in"),
         (3, Is, "is"),
-        (6, Match, "match")
+        (6, Match, "match"),
+        (12, Underscore, "_")
     );
 
     test_tokens!(

fine/tests/expression/is.fine

@@ -12,6 +12,9 @@ fun test() -> f64 {
   if b is c:Foo and c.a == 24 {
     result = result + 10;
   }
+  if b is c:_ {
+    result = result + 100; // underscore should always match!
+  }
   if b is c:Foo {
     result = result + c.a; // c should still be in scope!
   }
@@ -19,4 +22,4 @@ fun test() -> f64 {
 }
 
 // @no-errors
-// @eval: Float(1001.0)
+// @eval: Float(1101.0)