| 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@haskell.org] On Behalf Of Alan | & Kim Zimmerman | Sent: 20 December 2015 18:55 | To: ghc-devs@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 | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | 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