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| author | David Terei <davidterei@gmail.com> | 2011-07-20 11:09:03 -0700 |
|---|---|---|
| committer | David Terei <davidterei@gmail.com> | 2011-07-20 11:26:35 -0700 |
| commit | 16514f272fb42af6e9c7674a9bd6c9dce369231f (patch) | |
| tree | e4f332b45fe65e2a7a2451be5674f887b42bf199 /testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs | |
| parent | ebd422aed41048476aa61dd4c520d43becd78682 (diff) | |
| download | haskell-16514f272fb42af6e9c7674a9bd6c9dce369231f.tar.gz | |
Move tests from tests/ghc-regress/* to just tests/*
Diffstat (limited to 'testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs')
| -rw-r--r-- | testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs b/testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs new file mode 100644 index 0000000000..0117b81d47 --- /dev/null +++ b/testsuite/tests/indexed-types/should_compile/PushedInAsGivens.hs @@ -0,0 +1,24 @@ +{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, TypeFamilies #-}
+module PushedInAsGivens where
+
+
+type family F a
+
+
+
+bar y = let foo :: (F Int ~ [a]) => a -> Int
+ foo x = length [x,y]
+ in (y,foo y)
+
+
+-- This example demonstrates why we need to push in
+-- an unsolved wanted as a given and not a given/solved.
+-- [Wanted] F Int ~ [beta]
+--- forall a. F Int ~ [a] => a ~ beta
+-- We we push in the [Wanted] as given, it will interact and solve the implication
+-- constraint, and finally we quantify over F Int ~ [beta]. If we push it in as
+-- Given/Solved, it will be discarded when we meet the given (F Int ~ [a]) and
+-- we will not be able to solve the implication constraint.
+
+
+
|