[Haskell-cafe] How to get a list of constructors for a type?
claudiusmaximus at goto10.org
Wed Dec 30 15:46:43 EST 2009
Gregory Propf wrote:
> Say I have something like
> data DT a = Foo a | Bar a | Boo a
> I want something like a list of the constructors of DT, perhaps as [TypeRep]. I'm using Data.Typeable but can't seem to find what I need in there. Everything there operates over constructors, not types.
The approach below won't work in general because the constructors may
have different (argument) types, but for the above:
[Foo, Bar, Boo] :: [a -> DT a]
And you could manually build a list of constructor types, as a try:
[typeOf Nothing, typeOf Just] :: [TypeRep]
which doesn't work, due to polymorphic ambiguity, so you'd have to:
[typeOf (Nothing :: Maybe Int), typeOf (Just :: Int -> Maybe Int)]
In general, maybe Data.Dynamic is what you need, but I don't think that
can handle polymorphic values yet either.
What do you want to do with the list of constructors once you have them?
Where are they defined?
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