#9893: Switching on TypeFamilies extension stops code from typechecking -------------------------------------+------------------------------------- Reporter: phischu | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: x86_64 (amd64) Unknown/Multiple | Difficulty: Unknown Type of failure: GHC | Blocked By: rejects valid program | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Description changed by phischu: Old description:

The following code has the `TypeFamilies` extension enabled but does not use it. It does not typecheck with GHC 7.8.3. If we do not enable `TypeFamilies` (i.e. delete the first line) it typechecks.

{{{ {-# LANGUAGE TypeFamilies #-} module ExistentialFamilies where

import Control.Monad.ST (runST,ST)

un :: () un = runST f where f = return un :: ST s () }}} This happens when trying to compile the program as well as when trying to load it into ghci.

New description: The following code has the `TypeFamilies` extension enabled but does not use it. It does not typecheck with GHC 7.8.3. If we do not enable `TypeFamilies` (i.e. delete the first line) it typechecks. {{{ {-# LANGUAGE TypeFamilies #-} module ExistentialFamilies where import Control.Monad.ST (runST,ST) un :: () un = runST f where f = return un :: ST s () }}} This happens when trying to compile the program as well as when trying to load it into ghci. The error message when `TypeFamilies` is on is {{{

ghc ExistentialFamilies.hs [1 of 1] Compiling ExistentialFamilies ( ExistentialFamilies.hs, ExistentialFamilies.o )

ExistentialFamilies.hs:7:12: Couldn't match type ‘s0’ with ‘s’ because type variable ‘s’ would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: ST s () at ExistentialFamilies.hs:7:6-12 Expected type: ST s () Actual type: ST s0 () Relevant bindings include f :: ST s0 () (bound at ExistentialFamilies.hs:8:5) In the first argument of ‘runST’, namely ‘f’ In the expression: runST f }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9893#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler