Pattern synonym causes Iface error.

From the mailing list..

Hello, We have a pattern synonym as follows

type family Showable (a :: k) :: Constraint where
  Showable (a :: *) = (Show a)
  Showable a       = ()

pattern Just' :: () => (Showable a) => a -> (Maybe a)
pattern Just' a <- (extractJust -> (True, a)) where
  Just' a = Just a

When we try to use the pattern in a different package, the error was

    [1 of 1] Compiling Bar              ( Bar.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.4.0/build/Bar.o )
    /tmp/test/p2/.stack-work/install/x86_64-linux/lts-3.5/7.10.2/lib/x86_64-linux-ghc-7.10.2/p1-0.1.0.0-I5t4il6dN7vIqsT1XgYsM3/Foo.hi
    Declaration for Just'
    Pattern synonym Just':
      Iface type variable out of scope:  k
    Cannot continue after interface file error

The error only occurred when Showable was polykinded and we used synonym in a different package . Using the synonym in the same package works fine.

This problem did not happen with the following definition of (non polykinded ) Showable

type family Showable a :: Constraint where
  Showable a = (Show a)

changed weight to 5

added Tbug Trac import labels

changed the description

Does this amount to a reproducible test case? What is Bar.hs?

Attached file bug.tar.gz (download).

Yes, see the attached file and run "stack build" in p2/ is one way.

Another way to trigger it is to have, two files Foo.hs and Bar.hs as follows. Then run the following commands..

ghc Foo.hs
ghc Bar.hs

[2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
The interface for ‘Foo’
Declaration for Just'
Pattern synonym Just':
  Iface type variable out of scope:  k
Cannot continue after interface file error

{-# LANGUAGE PatternSynonyms, ViewPatterns, ConstraintKinds, TypeFamilies, PolyKinds, KindSignatures #-}
module Foo where

import GHC.Exts

type family Showable (a :: k) :: Constraint where
  Showable (a :: *) = (Show a)
  Showable a       = ()

extractJust :: Maybe a -> (Bool, a)
extractJust (Just a) = (True, a)
extractJust _        = (False, undefined)

pattern Just' :: () => (Showable a) => a -> (Maybe a)
pattern Just' a <- (extractJust -> (True, a)) where
  Just' a = Just a

module Bar where

import Foo

bar :: (Showable a) => Maybe a -> Maybe a
bar (Just' a) = Just' a

mentioned in commit 68a084f3

Trac metadata

Trac field	Value
Test case	- → typecheck/should_compile/T10997

I am also affected by this bug when using -fobject-code.

Given

{-# LANGUAGE GADTs, PatternSynonyms #-}

module Foo where

data Exp ty where
  LitB :: Bool -> Exp Bool

pattern Tru :: b ~ Bool => Exp b
pattern Tru = LitB True

and

module Bar where

import Foo

foo :: Exp a -> String
foo Tru = "True"

results in the following session:

% ghci -ignore-dot-ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> :load Bar
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Bar              ( Bar.hs, interpreted )
Ok, modules loaded: Bar, Foo.
*Bar> :set -fobject-code 
*Bar> :load Foo
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
Ok, modules loaded: Foo.
Prelude Foo> :load Bar
[2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
The interface for ‘Foo’
Declaration for Tru
Pattern synonym Tru:
  Iface type variable out of scope:  k
Cannot continue after interface file error
> 

Also simply triggered by:

% ghc Foo      
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
% ghc Bar
[2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
The interface for ‘Foo’
Declaration for Tru
Pattern synonym Tru:
  Iface type variable out of scope:  k
Cannot continue after interface file error

which causes some tension in my own projects where I use pattern synonyms generously.

changed weight to 7

Trac metadata

Trac field	Value
Priority	normal → high

mentioned in commit 0ce858e5

Thanks for reporting this.

Trac metadata

Trac field	Value
Test case	typecheck/should_compile/T10997 → patsyn/should_compile/T10997, T10997_1

added 1 deleted label

changed milestone to %7.10.3

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Phigh label

mentioned in merge request !8612 (merged)