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Diffstat (limited to 'testsuite/tests/typecheck/should_fail/tcfail076.hs')
| -rw-r--r-- | testsuite/tests/typecheck/should_fail/tcfail076.hs | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_fail/tcfail076.hs b/testsuite/tests/typecheck/should_fail/tcfail076.hs new file mode 100644 index 0000000000..abe96c5640 --- /dev/null +++ b/testsuite/tests/typecheck/should_fail/tcfail076.hs @@ -0,0 +1,30 @@ +{-# LANGUAGE Rank2Types #-} + +{- + From: Ralf Hinze <ralf@uran.informatik.uni-bonn.de> + Date: Fri, 15 Aug 1997 15:20:51 +0200 (MET DST) + +I *suppose* that there is a bug in GHC's type checker. The following +program, which I think is ill-typed, passes silently the type checker. +Needless to say that it uses some of GHC's arcane type extensions. +-} + +module ShouldFail where + +data ContT m a = KContT (forall res. (a -> m res) -> m res) +unKContT (KContT x) = x + +callcc :: ((a -> ContT m b) -> ContT m a) -> ContT m a +callcc f = KContT (\cont -> unKContT (f (\a -> KContT (\cont' -> cont a))) cont) + +{- +`ContT' is a continuation monad transformer. Note that we locally +qualify over the result type `res' (sometimes called answer or +output). IMHO this make it impossible to define control constructs +like `callcc'. Let's have a closer look: the code of `callcc' contains +the subexpression `KContT (\cont' -> cont a)'. To be well-typed the +argument of `KContT' must have the type `(All res) => (a -> m res) -> m +res'. Quantification is not possible, however, since the type variable +in `cont's type cannot be forall'd, since it also appears at an outer +level. Right? Or wrong? +-} |