[GHC] #12643: class declaration works in ghci, but not in a file
#12643: class declaration works in ghci, but not in a file -------------------------------------+------------------------------------- Reporter: dmwit | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following ghci session prints no errors: {{{
:set -XStandaloneDeriving -XDeriveGeneric import GHC.Generics import Control.Monad.Except deriving instance Generic (ExceptT e m a) class F a where f :: Rep (Except String a) x }}}
However, when I transfer this to a file: {{{ {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE DeriveGeneric #-} import GHC.Generics import Control.Monad.Except deriving instance Generic (ExceptT e m a) class F a where f :: Rep (Except String a) x }}} I get a mysterious error: {{{ test.hs:6:17: error: • Couldn't match type ‘Rep (Except String a0)’ with ‘Rep (Except String a)’ Expected type: Rep (Except String a) x Actual type: Rep (Except String a0) x NB: ‘Rep’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous • In the ambiguity check for ‘f’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the class method: f :: forall a. F a => forall x. Rep (Except String a) x In the class declaration for ‘F’ }}} If I turn on -fdefer-type-errors, I can verify that the type family indeed reduces far enough for a and x to be arguments to injective types, so I believe GHC should not consider this an error: {{{
:set -XRankNTypes :kind! forall a x. Rep (Except String a) x forall a x. Rep (Except String a) x :: * = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.5.2.0" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (Data.Functor.Identity.Identity (Either [Char] a))))) x }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12643 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12643: class declaration works in ghci, but not in a file -------------------------------------+------------------------------------- Reporter: dmwit | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Crumbs. This is another variation of #11348, and its as-yet-unsolved cousin #12088. In this case, we must type check in this order: 1. `deriving instance Generic (ExceptT e m a)`. This generates a `type instance Rep (Except e m a) = ...` declaration, for `Generic`'s associated type `Rep`. 2. `class F a where ...`. We must do this second because `f`'s type can be seen to be unambiguous only after expanding the call to `Rep`. However the fix to #11348 arranges to interleave `type instance` declarations with type/class decls; but `deriving instance` declarations are still left to the end. The fix is, I think, to include `deriving instance` decls in the interleaving. Workaround for now: make the staging explicit with a degenerate Template Haskell splice, thus: {{{ deriving instance Generic (ExceptT e m a) $( return [] ) -- Degenerate splice class F a where f :: Rep (Except String a) x }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12643#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12643: class declaration works in ghci, but not in a file -------------------------------------+------------------------------------- Reporter: dmwit | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: RyanGlScott (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12643#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12643: class declaration works in ghci, but not in a file -------------------------------------+------------------------------------- Reporter: dmwit | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12088, #14668, | Differential Rev(s): #15561, #15987 | Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #12088, #14668, #15561, #15987 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12643#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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