[Haskell-cafe] How to get a list of constructors for a type?

[Lennart Augustsson]

I've put some of Oleg's code on hackage, named polytypeable.

> import Data.PolyTypeable
> main = print [polyTypeOf Nothing, polyTypeOf Just]

This prints
  [Maybe a1,a1 -> Maybe a1]

To get a list of the actual constructors you need to derive
Data.Data.Data and use that.

  -- Lennart

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?
>
>
> Claude
> --
> http://claudiusmaximus.goto10.org
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