Change from 7.10.3 to current master

[Alan & Kim Zimmerman]

This is now https://ghc.haskell.org/trac/ghc/ticket/11307

On Mon, Dec 21, 2015 at 1:03 PM, Simon Peyton Jones
<simonpj at microsoft.com> wrote:
> |  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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