conflicting multi-parameter family instance declarations

[Simon Peyton Jones]

Alas it's deliberate.  See Section 6 of "Closed type families" http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/, and the recent thread on https://ghc.haskell.org/trac/ghc/ticket/9918

Maybe you can add your example to that ticket, with some indication of why it's important to you.

The difficulty is that lifting this restriction actually makes the type system unsound.

Simon

|  -----Original Message-----
|  From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Michal Konecný
|  Sent: 13 January 2015 12:08
|  To: glasgow-haskell-users at haskell.org
|  Subject: conflicting multi-parameter family instance declarations
|  
|  Dear all,
|  
|  The following compiles with ghc 7.6 but fails with ghc 7.8:
|  
|  -----
|  {-# LANGUAGE TypeFamilies #-}
|  {-# LANGUAGE FlexibleInstances #-}
|  {-# LANGUAGE MultiParamTypeClasses #-}
|  module Test where
|  
|  class M t s where
|      type T t s
|  
|  data I t = I t
|  
|  instance M t t where
|      type (T t t) = ()
|  
|  instance M t (I t) where
|      type (T t (I t)) = ()
|  -----
|  
|  The error I get with ghc 7.8.3 and 7.8.4 is:
|  
|  Test.hs:12:10:
|      Conflicting family instance declarations:
|        T t t -- Defined at Test.hs:12:10
|        T t (I t) -- Defined at Test.hs:15:10
|  
|  
|  I am curious if this change is an improvement or a bug.  I would be
|  grateful for help as this issue affects a fairly large library I
|  develop.
|  
|  Best regards,
|  Michal
|  --
|  |-| Dr. Michal Konecny, Computer Science, Aston University Room MB212D
|  |
|  |-| Tel +44 121 204 3462 | Fax +44 121 204 3681
|  |-| http://duck.aston.ac.uk/konecnym OpenPGP key
|  |-| http://duck.aston.ac.uk/konecnym/ki.aston
|  
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