Comments only

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index ee62c0e..a6bae24 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2237,12 +2237,29 @@ We can satisfy the (C Int) from the superclass of D, so we don't want
 to float the (C Int) out, even though it mentions no type variable in
 the constraints!
 
 to float the (C Int) out, even though it mentions no type variable in
 the constraints!
 

+One more example: the constraint
+       class C a => D a b
+       instance (C a, E c) => E (a,c)
+
+       constraint: forall b. D Int b => E (Int,c)
+
+You might think that the (D Int b) can't possibly contribute
+to solving (E (Int,c)), since the latter mentions 'c'.  But 
+in fact it can, because solving the (E (Int,c)) constraint needs 
+dictionaries
+       C Int, E c
+and the (C Int) can be satisfied from the superclass of (D Int b).
+So we must still not float (E (Int,c)) out.
+
+To think about: special cases for unary type classes?
+

 Note [Pruning the givens in an implication constraint]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we are about to form the implication constraint
        forall tvs.  Eq a => Ord b
 The (Eq a) cannot contribute to the (Ord b), because it has no access to
 the type variable 'b'.  So we could filter out the (Eq a) from the givens.
 Note [Pruning the givens in an implication constraint]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we are about to form the implication constraint
        forall tvs.  Eq a => Ord b
 The (Eq a) cannot contribute to the (Ord b), because it has no access to
 the type variable 'b'.  So we could filter out the (Eq a) from the givens.

+But BE CAREFUL of the examples above in [Freeness and implications].

 
 Doing so would be a bit tidier, but all the implication constraints get
 simplified away by the optimiser, so it's no great win.   So I don't take
 
 Doing so would be a bit tidier, but all the implication constraints get
 simplified away by the optimiser, so it's no great win.   So I don't take