Re: [GHC] #12387: Template Haskell ignores class instance definitions with methods that don't belong to the class

17 May 2018

      #12387: Template Haskell ignores class instance definitions with methods that don't
belong to the class
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 To address my hunch in the original description, it turns out that the
 same issue does //not// affect associated type family instances. That is,
 if you try compiling this program:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Main where

 import GHC.Generics
 import Language.Haskell.TH.Lib

 data Foo = Foo

 $(do d <- instanceD (cxt []) (conT ''Eq `appT` conT ''Foo)
             [tySynInstD ''Rep $ tySynEqn [conT ''Foo] (conT ''Maybe)]
      return [d])

 main :: IO ()
 main = print $ Foo == Foo
 }}}

 Then it will rightly complain about `Rep` not being an associated type  of
 `Eq`:

 {{{
 $ /opt/ghc/8.4.2/bin/ghc Bug.hs
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Bug.hs:(11,3)-(13,15): Splicing declarations
     do d_a2Bz <- instanceD
                    (cxt [])
                    (conT ''Eq `appT` conT ''Foo)
                    [tySynInstD ''Rep $ tySynEqn [conT ''Foo] (conT
 ''Maybe)]
        return [d_a2Bz]
   ======>
     instance Eq Foo where
       type Rep Foo = Maybe

 Bug.hs:11:3: error:
     • Class ‘Eq’ does not have an associated type ‘Rep’
     • In the type instance declaration for ‘Rep’
       In the instance declaration for ‘Eq Foo’
    |
 11 | $(do d <- instanceD (cxt []) (conT ''Eq `appT` conT ''Foo)
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 That's because this particular validity check is performed
 [http://git.haskell.org/ghc.git/blob/5f15d53a98ad2f26465730d8c3463ccc58f6d94a...
 during typechecking], not renaming. This makes me believe that postponing
 the analogous check for class methods would also be wise (i.e., simonpj's
 second bullet point in comment:1).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12387#comment:3
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