type equality symbol
Simon PeytonJones
simonpj at microsoft.com
Wed Dec 5 11:05:22 EST 2007
 > Nothing deep. Just that "=" means so many things that it seemed better
 > to use a different notation.
 >

 How about ==? Only one meaning so far, and that both on the term level and
 equivalent to the constraint
I'm quite happy with "~"! It's sufficiently different from "=" that someone meeting it for the first time is going to thing "hmm, better read the manual"; and they'd be right.
Anyway, while on this subject, I am considering making the following change:
make all operator symbols into type constructors
(currently they are type variables)
That would allow you to write
data a * b = Prod a b
data a + b = Left a  Right b
and write naturallooking types like
f :: a*b > a
When we have indexed type families working, this will be even more natural.
As things stand, only operators starting with a ":" are type constructors, thus
data a :*: b = Prod a b
etc. By analogy with the term language, operators are currently classified as "type variables", so you could write (oddly)
data T (+) x = MkT (+) x
[this may not even work today, but it should] to mean the same as
data T y x = MkT y x
But this is pretty useless! Very occasionally one might want a type variable with kind (*>*>*), but much much more often you want a type *constructor* with that kind.
I thought I'd mention this here in case people have ideas.
Simon
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