| 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:[email protected]] On Behalf Of Alan
| & Kim Zimmerman
| Sent: 20 December 2015 18:55
| To: [email protected]
| 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
| _______________________________________________
| ghc-devs mailing list
| [email protected]
| https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.h
| askell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc-
| devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c17b33d7be13045d
| d141308d3096f2bc5%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=LGroqdv
| nYFSxydfZFQydhi2N6lltWwi%2b4tt%2bM3LH0P0%3d
_______________________________________________
ghc-devs mailing list
[email protected]
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
