Mon, 29 Jul 2002 21:25:40 +0100
Ken Shan <firstname.lastname@example.org> wrote:
> In Haskell, backquotes can be used to convert individual identifiers
> into infix operators, but not complex expressions. For example,
> [1,2,3] `zip` [4,5,6]
> is OK, but not
> [1,2,3] `zipWith (+)` [4,5,6]
> Is there any reason other than potential confusion when one of the two
> backquotes is accidentally omitted?
I've often wondered about this myself, but it's difficult to
make a pleasant distinction between what's allowed in
between `` and an ordinary expression. They can't be the
same because you can't nest them. Using a matched pair of
quotation marks would work, but then you have the
possibility of writing really horrid expressions.
> In any case, perhaps some people on this mailing list would appreciate
> the following implementation of "infix expressions" that Dylan Thurston=
> and I came up with -- as algebraic and perverse as we could manage:
> infixr 0 -:, :-
> data Infix f y =3D f :- y
> x -:f:- y =3D x `f` y
> main =3D print $ [1,2,3] -: zipWith (+) :- [4,5,6]
Yes, I appreciate that! It reminds me of how I got the
syntax of Ponder -- which had no predefined operators, not
even "if" -- to work.
> The trick is that there is no trick.
Oh, I think it /is/ a trick :-)
J=F3n Fairbairn Jon.Fairbairn@cl.cam.ac.u=
31 Chalmers Road email@example.com
Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)