On 3 Jan 2010, at 05:18, Daniel Fischer <daniel.is.fisc...@web.de>
wrote:
Am Sonntag 03 Januar 2010 05:37:31 schrieb Jason Dusek:
> Well, you can, with:
>
> -XTypeSynonymInstances
>
> though I'm not sure it addresses your specific need.
Doesn't help him here, he would need
instance Monad (State s) where ...
but that would be a partially applied type synonym. He would also
need type level lambdas,
type State s = /\ a -> (s -> (a,s))
But type level lambdas and partially applied type synonyms make type
inference undecidable if I remember correctly (if it wasn't that,
they'd have other dire consequences).
Type inference is undecidable already. It's still extremely useful,
cut with just enough type annotation and checking to resolve
ambiguities. That creates a bunch of trade-offs to negotiate and a
design space to explore.
Lots of dependent type systems have type-level lambda (that is, they
have lambda), undecidable type inference, decidable type checking, and
substantial (but inadequately rationalised) facilities for suppressing
and recovering boring details. Nobody's campaigning to kick type-level
lambda out of Agda.
Adding type-level lambda to Haskell would amount to admitting the
awful truth: application is not injective. When you write a do-program
of type t, you need to solve a constraint m x = t for m and x, to
figure out which monad to plumb in. Pretending application is
injective makes this easy, at the cost of requiring lots of wrapper-
types. Type-level lambda makes this constraint highly ambiguous: Maybe
Int is also (\ x -> x) (Maybe Int), the type of a computation in the
identity monad. To cope, we'd need a new way to be signal such
decompositions. Worth thinking about, perhaps, but certainly a Big Deal.
Type-level lambda is not inherently disastrous. It's just a poor fit
with Haskell's current design.
Cheers
Conor
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