Commit 25f8fc63 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Remove duplicate comment

parent 44c7a4c6
......@@ -984,39 +984,6 @@ thBrackId orig id_name id ps_var lie_var
#endif /* GHCI */
\end{code}
Note [Multiple instantiation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We are careful never to make a MethodInst that has, as its meth_id, another MethodInst.
For example, consider
f :: forall a. Eq a => forall b. Ord b => a -> b
At a call to f, at say [Int, Bool], it's tempting to translate the call to
f_m1
where
f_m1 :: forall b. Ord b => Int -> b
f_m1 = f Int dEqInt
f_m2 :: Int -> Bool
f_m2 = f_m1 Bool dOrdBool
But notice that f_m2 has f_m1 as its meth_id. Now the danger is that if we do
a tcSimplCheck with a Given f_mx :: f Int dEqInt, we may make a binding
f_m1 = f_mx
But it's entirely possible that f_m2 will continue to float out, because it
mentions no type variables. Result, f_m1 isn't in scope.
Here's a concrete example that does this (test tc200):
class C a where
f :: Eq b => b -> a -> Int
baz :: Eq a => Int -> a -> Int
instance C Int where
baz = f
Current solution: only do the "method sharing" thing for the first type/dict
application, not for the iterated ones. A horribly subtle point.
%************************************************************************
%* *
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