[MLton-user] Wish for a standard sum type

[Jesper Louis Andersen]

On Sun, 2006-05-28 at 14:41 +0300, Vesa Karvonen wrote:
> The Standard ML Basis Library doesn't contain a general purpose sum type
> like the Either datatype provided by the Haskell prelude:
> 
>   data Either a b = Left a
>                   | Right b
> 
> I have personally found such a general purpose sum type highly useful, and
> would definitely like to see one in the Basis Library at some point.
> 
> The obvious translation to SML would be
> 
>   datatype ('a, 'b) either = LEFT of 'a
>                            | RIGHT of 'b

datatype ('a, 'b) either = INL of 'a
                         | INR of 'b

datatype ('a, 'b) either = LEFT of 'a
                         | RITE of 'b   (Terry Pratchett fans will love)

L | R, A | B is much too general. INL | INR is grateously stolen from a
couple of courses on formal semantics. LEFT | RITE is for fun and play.
I do find it on the scale of THIS | THAT. Of course you define:

inl x = INL x 
inr x = INR x 

to match it up with the formal semantics more.

j.