[GHC] #11307: Regresssion: parsing type operators

28 Dec 2015


      #11307: Regresssion: parsing type operators
-------------------------------------+-------------------------------------
           Reporter:  alanz          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.11
           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:
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 GHC 7.10.3 will parse (but not type-check) the following

 {{{#!hs
 {-# LANGUAGE TypeOperators #-}

 type family (r1 :++: r2); infixr 5 :++:
 type instance r :++: Nil = r
 type instance r1 :++: r2 :> a = (r1 :++: r2) :> a
 }}}

 Current master (c7830bdb14e0a85a78617314156d101155fdf7aa) fails with

 {{{#!hs
 /tmp/Foo.hs:5:15: error:
     Malformed head of type or class declaration: r1 :++: r2 :> a
 }}}

 @simonpj comment on ghc-devs mailing list
...
`|  type instance r1 :++: r2 :> a = (r1 :++: r2) :> a`
What do you expect this to mean?  I suppose you could hope that GHC will
unravel the fixity of :++: and :>, to determine whether you are giving
 an
instance for :++: or for :>?
That sounds reasonable, but it's not trivial.
Currently the LHS of a 'type instance' decl is ultimately a TyFamEqn,
 and
it looks like this:
{{{#!hs
data TyFamEqn name pats
  = TyFamEqn
       { tfe_tycon :: Located name
       , tfe_pats  :: pats
       , tfe_rhs   :: LHsType name }
}}}
So we've already decided (in the parser) what the type-function name is.
But we can't do that in this case, because the parser doesn't understand
fixity.
To deal with this we'd need to change to
{{{#!hs
data TyFamEqn name pats
  = TyFamEqn
       { tfe_lhs  :: LHSType name
       , tfe_rhs   :: LHsType name }
}}}
so that the LHS was just a type.  Now the renamer can re-jiggle its
fixities in the usual way, and only in the type checker will we need to
decide exactly which type it's an instance of.  Easy!
This might be a good change to make! It's a bit like in ClsInstDecl,
 you'll
see that the instance cid_poly_ty is just a LHsSigType, i.e. not
 decomposed
into which class it is.
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11307
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