infix type constructors
Thu, 16 May 2002 07:24:10 -0700
I'm slowly getting around to this. Design questions:
(A) I think it would be a good compromise to declare that operators
like "+" are type *constructors* not type *variables*. So=20
would be a type. That's slightly inconsistent with value variables,
but it's jolly useful. So only alphabetic things would be type
It's very clunky having to write
S :+: T
(B) One wants to declare fixities for type constructors, and that
gets them mixed up with their value counterparts. My suggestion:
disamiguate with a compulsory 'type' keyword
infix 6 type +
infixl 9 type *
Or should it be 'data'? Or should it depend how + and * are declared?
(C) The other place they can get mixed up is in import and export
lists. I can think of several solutions
(i) module Foo( + ) where ...
means export the type constructor (+); currently illegal in H98
module Foo( (+) ) where ...
means export the variable (+).
This seems a bit of a hack.
(ii) Use the 'type' keyword, rather like 'module':
module Foo( type + ) where=20
data a+b =3D A a | B b
module Foo( type +(A,B) ) where
data a+b =3D A a | B b
[I think 'type' is better than 'data' because we want to hide the=20
distinction in an export list.... or do you think we should use the=20
same keyword as the one in the defn?]
Similarly on import lists.
(D) I suppose one might want infix notation for type variables too:
data T a =3D T (Int `a` Int)
but maybe that's going too far?
| -----Original Message-----
| From: Okasaki, C. DR EECS [mailto:Christopher.Okasaki@usma.edu]=20
| Sent: 03 May 2002 14:09
| To: 'email@example.com'
| Subject: infix type constructors
| I'm not sure how long this has been implemented in GHC,
| but I just noticed that infix type constructors are allowed,
| as in
| data a :- b =3D ...
| The syntactic asymmetry between type constructors and
| data contructors has bothered me for a while, so this
| is a welcome change! However, this syntax seems to
| be supported for "data" and "newtype" declarations,
| but not for "type" declarations. For example,
| type a :- b =3D ...
| does not seem to be allowed. Is there a reason for this?
| Or was it just an oversight?
| -- Chris
| Glasgow-haskell-users mailing list=20
| http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users