Fix #215

Author: jfecher

examples/typechecking/effects_subset.an

@@ -0,0 +1,32 @@
+// Regression for #215
+effect Emit a with
+    emit: a -> Unit
+
+iota (n: U32) = loop (i = 0) ->
+    if i < n then
+        emit i
+        recur (i + 1)
+
+filter stream predicate =
+    handle stream ()
+    | emit x ->
+        if predicate x then
+            emit x
+        resume ()
+
+for stream f =
+    handle stream ()
+    | emit x ->
+        f x
+        resume ()
+
+iota 10
+    with filter (fn x -> x % 2 == 0)
+    with for print
+
+// args: --check --show-types
+// expected stdout:
+// emit : forall a. (a -> Unit can Emit a)
+// filter : forall a b c d e. ((Unit => a can Emit d, e) - (d => Bool can Emit d, e) -> a can Emit d, e)
+// for : forall a b c d e f. ((Unit => a can Emit c, f) - (c => d can f) -> a can f)
+// iota : U32 -> Unit can Emit U32

src/types/effects.rs

@@ -149,11 +149,11 @@ impl EffectSet {
     }
 
     pub fn try_unify_with_bindings<'c>(
-        &self, other: &EffectSet, bindings: &mut UnificationBindings, location: Location<'c>,
+        &self, expected: &EffectSet, bindings: &mut UnificationBindings, location: Location<'c>,
         cache: &mut ModuleCache<'c>,
     ) -> Result<(), ()> {
         let a = self.follow_unification_bindings(bindings, cache);
-        let b = other.follow_unification_bindings(bindings, cache);
+        let b = expected.follow_unification_bindings(bindings, cache);
 
         let mut new_effects_in_a = Vec::new();
         let mut new_effects_in_b = Vec::new();
@@ -185,8 +185,10 @@ impl EffectSet {
         check_effects(&a.effects, &b.effects, &mut new_effects_in_b);
         check_effects(&b.effects, &a.effects, &mut new_effects_in_a);
 
-        if a.extension.is_none() && !new_effects_in_a.is_empty()
-            || b.extension.is_none() && !new_effects_in_b.is_empty()
+        // Allow extra effects in `a` (actual) if `b` (expected) has extra,
+        // but not the reverse. This allows, e.g. passing a pure function
+        // into a function expecting a function with an effect.
+        if b.extension.is_none() && !new_effects_in_b.is_empty()
         {
             return Err(());
         }
@@ -208,7 +210,11 @@ impl EffectSet {
             }
         };
 
-        let a_extension = extend_effects(new_effects_in_a, a.extension)?;
+        let a_extension = match a.extension {
+            Some(extension) => extend_effects(new_effects_in_a, Some(extension))?,
+            None => None,
+        };
+
         let b_extension = extend_effects(new_effects_in_b, b.extension)?;
 
         if let (Some(a), Some(b)) = (a_extension, b_extension) {