Dear GHC users
As part of beefing up the kind system, we plan to implement the "Type
operators" proposal for Haskell Prime
http://hackage.haskell.org/trac/haskell-prime/wiki/InfixTypeConstructors
GHC has had type operators for some kind, so you can say
data a :+: b = Left a | Right b
but you can only do that for operators which start with ":".
As part of the above wiki page you can see the proposal to broaden this to ALL
operators, allowing
data a + b = Left a | Right b
Although this technically inconsistent the value page (as the wiki page
discussed), I think the payoff is huge. (And "A foolish consistency is the
hobgoblin of little minds", Emerson)
This email is (a) to highlight the plan, and (b) to ask about flags. Our
preferred approach is to *change* what -XTypeOperators does, to allow type
operators that do not start with :. But that will mean that *some* (strange)
programs will stop working. The only example I have seen in tc192 of GHC's test
suite
{-# LANGUAGE TypeOperators #-}
comp :: Arrow (~>) => (b~>c, c~>d)~>(b~>d)
comp = arr (uncurry (>>>))
Written more conventionally, the signature would look like
comp :: Arrow arr => arr (arr b c, arr c d) (arr b d)
comp = arr (uncurry (>>>))
or, in infix notation
{-# LANGUAGE TypeOperators #-}
comp :: Arrow arr => (b `arr` c, c `arr` d) `arr` (b `arr` d)
comp = arr (uncurry (>>>))
But tc192 as it stands would become ILLEGAL, because (~>) would be a type
*constructor* rather than (as now) a type *variable*. Of course it's easily
fixed, as above, but still a breakage is a breakage.
It would be possible to have two flags, so as to get
- Haskell 98 behaviour
- Current TypeOperator behaviuor
- New TypeOperator behaviour
but it turns out to be Quite Tiresome to do so, and I would much rather not.
Can you live with that?
http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.html
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