[Haskell-cafe] understanding typeable
dan.doel at gmail.com
Sun Apr 12 18:07:18 EDT 2009
On Sunday 12 April 2009 3:42:35 pm Anatoly Yakovenko wrote:
> i am trying to figure out how typeable works, so i have this data type
> data Foo = FooC Int
> deriving (Data, Typeable, Show)
> So how come this works:
> > funResultTy (typeOf ((+) 1)) (typeOf 1)
> Just Integer
> but this doesnt:
> > funResultTy (typeOf FooC) (typeOf 1)
> FooC is of type t -> u and 1 is of type t so the result should be u?
> I don't think t and u need to be the same since
> > funResultTy (typeOf (\a -> a:)) (typeOf 1)
> Just [Integer]
> works fine. So given a constructor, how come i cant seem to construct
> a type out of it?
Judging from the other types, 'typeOf 1' is defaulting to Integer. FooC, per
your declaration, has type 'Int -> Foo'. So, you're probably asking it what
happens when you apply an Int -> Foo to an Integer, which would be a type
error, and explains the Nothing, I think.
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