#13249: Default signature check can be quite onerous -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #12918 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: @@ -15,1 +15,1 @@ - (t m)) + (t n)) New description: The `bytes` library currently doesn't compile with GHC `master` due to the new check of default signatures (7363d5380e600e2ef868a069d5df6857d9e5c17e, #12918). Consider this example, {{{#!hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DefaultSignatures #-} module Hi where import Control.Monad.Trans.Class class MonadGet m where type Remaining m :: * remaining :: m (Remaining m) default remaining :: (MonadTrans t, MonadGet n, m ~ t n) => m (Remaining (t n)) remaining = lift remaining }}} Patching this up requires a fair amount of hand-holding, {{{#!hs default remaining :: (MonadTrans t, MonadGet n, m ~ t n, Remaining m ~ Remaining n, Monad n) => m (Remaining m) }}} Yuck. I suppose this is just how the world works, but I thought I'd leave this here in case anyone had a clever idea for improvement. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13249#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler