My original question was related to use of existing facilities. I find it somewhat surprising how rich and complex a language Haskell turns out to be. I'm wary of yet more features. That said, you seem to be suggesting a kind of "anonymous" union type (to complement the proposed "anonymous" record type?). At one level, that seems to answer the question I was raising, and seems to require that alternative selection is based on (unification of?) the datatypes, rather than by simple matching of constructor labels. That sounds to me a bit like class instance selection (and I note that Scott did suggest using type classes). Could your proposal be viewed as a syntactic sugaring of classes and instances, without explicit instance declarations? #g -- At 10:35 08/07/04 +0100, Alastair Reid wrote:
It sounds a little as though you want a family of n-ary union type constructor. A crude first attempt would be:
data Either2 t1 t2 = In1 t1 | In2 t2 data Either3 t1 t2 t3 = In1 t1 | In2 t2 | In3 t3 ...
but this would be a bit tedious to use because the names are a bit meaningless - you'd be relying heavily on type checking to keep you straight. e.g., it's kinds hard to read:
f :: Either String Bool -> Either String Bool f (In1 "Fred") = In1 "Frederick" f (In2 m) = In2 (not m)
Proposals for records gives you an easy way to define arbitrary product types with convenient syntax:
{ name = "Fred", married = True } :: { name :: String, married :: Bool }
Maybe what you really want here is a way to define union types with some convenient syntax. In the following made up syntax, |[ ... |] contains a list of types to be unioned together with each type labelled by a constructor name.
f :: |[ Name :: String, Married :: Bool |] -> |[ Name :: String, Married :: Bool |] f |[ Name = "Fred" |] = |[ Name = "Frederick |] f [| Married = m |] = [| Married = not m |]
I wonder if an extension like this would be generally useful? (Would it be useful for defining a compiler/interpreter by first giving a simple language and then adding further operations?)
-- Alastair Reid
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