[Haskell-cafe] What inhabits this type: (forall a. a -> b) ->
(forall a. m a -> m b)
Isaac Dupree
isaacdupree at charter.net
Tue Feb 27 18:20:18 EST 2007
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Jacques Carette wrote:
> Since my last query was answered so quickly, let's try another.
>
> I have looked on Hoogle. I would have asked Djinn, but I don't have it
> around. So, can someone find a term that inhabits
> (forall a. a -> b) -> (forall a. m a -> m b)
> ? I think of this as the type of functions that, given a function from
> any boxed-up a to a b, will give me a function from a boxed-up ma to a m
> b -- m does not have to be a Monad!.
Nothing but _|_, I would think -- m doesn't even have to be a Functor,
it could be
data Stupid a = Stupid
with no use for the argument (forall a. a -> b) and no way to know for
all 'm' how to make an 'm a' for some 'a'.
Isaac
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