[Haskell-cafe] How to get a list of constructors for a type?
lennart at augustsson.net
Wed Dec 30 17:51:27 EST 2009
I've put some of Oleg's code on hackage, named polytypeable.
> import Data.PolyTypeable
> main = print [polyTypeOf Nothing, polyTypeOf Just]
[Maybe a1,a1 -> Maybe a1]
To get a list of the actual constructors you need to derive
Data.Data.Data and use that.
On Wed, Dec 30, 2009 at 8:46 PM, Claude Heiland-Allen
<claudiusmaximus at goto10.org> wrote:
> 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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