Add comments about pruning implication constraints

[ghc-hetmet.git] / compiler / typecheck / TcSimplify.lhs

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 182a7c5..1689dab 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -848,6 +848,7 @@ bindIrredsR loc qtvs co_vars reft givens irreds
   | otherwise
   = do { let givens' = filter isDict givens
                -- The givens can include methods
+               -- See Note [Pruning the givens in an implication constraint]
 
           -- If there are no 'givens', then it's safe to 
           -- partition the 'wanteds' by their qtvs, thereby trimming irreds
@@ -1876,6 +1877,20 @@ 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!
 
+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.
+
+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
+advantage of that at the moment.
+
+If you do, BE CAREFUL of wobbly type variables.
+
+
 %************************************************************************
 %*                                                                     *
                Avails and AvailHow: the pool of evidence