Change from 7.10.3 to current master

[Simon Peyton Jones]

|  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:


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

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.


So: my instinct is that the above would be Fine Thing.  Make a ticket?  Add these comments to give context?  Excecute?

Thanks

Simon

|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Alan
|  & Kim Zimmerman
|  Sent: 20 December 2015 18:55
|  To: ghc-devs at haskell.org
|  Subject: Change from 7.10.3 to current master
|  
|  I am in the process of updating ghc-exactprint for current GHC master.
|  
|  One of the tests has the following in it
|  
|  -----------------------------
|  {-# LANGUAGE TypeOperators #-}
|  
|  type family (r1 :++: r2); infixr 5 :++:
|  type instance r :++: Nil = r
|  type instance r1 :++: r2 :> a = (r1 :++: r2) :> a
|  ------------------------------
|  
|  Current GHC master rejects this with
|  
|  /tmp/Foo.hs:5:15: error:
|      Malformed head of type or class declaration: r1 :++: r2 :> a
|  
|  Is this expected, or a bug?
|  
|  Alan
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