diff options
Diffstat (limited to 'testsuite/tests/indexed-types/should_fail/T4272.hs')
-rw-r--r-- | testsuite/tests/indexed-types/should_fail/T4272.hs | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/testsuite/tests/indexed-types/should_fail/T4272.hs b/testsuite/tests/indexed-types/should_fail/T4272.hs new file mode 100644 index 0000000000..3370fc3637 --- /dev/null +++ b/testsuite/tests/indexed-types/should_fail/T4272.hs @@ -0,0 +1,22 @@ + {-# LANGUAGE TypeFamilies, ScopedTypeVariables, FlexibleContexts #-} +module T4272 where + +class Family f where + terms :: f a -> a + +class Family (TermFamily a) => TermLike a where + type TermFamily a :: * -> * + +laws :: forall a b. TermLike a => TermFamily a a -> b +laws t = prune t (terms (undefined :: TermFamily a a)) + +prune :: TermLike a => TermFamily a a -> TermFamily a a -> b +prune = undefined + +-- terms :: Family f => f a -> a +-- Instantiate with f = TermFamily a +-- terms :: Family (TermFamily a) => TermFamily a a -> a +-- (terms (undefined::TermFamily a a) :: Family (TermFamily a) => a +-- So the call to prune forces the equality +-- TermFamily a a ~ a +-- which triggers an occurs check
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