[Haskell-beginners] how to feel free of the Implementation of Data structure

[Brent Yorgey]

On Thu, Mar 24, 2011 at 06:34:57PM +0800, 吴兴博 wrote:
> 
> 2.
> f1 :: (Integral a) => a -> String
> f2 :: (Integral a) => String -> a
> 
> pipe = f1 . f2
> It is clear that pipe is String -> String,

Actually, this code will give an "ambiguous type" error, since the
compiler cannot figure out what type 'a' should be.  And the type that
is chosen may make a difference for the behavior.

> but.(1)How can I know what instance that 'a' is.
>     (2)How can I determine what instance of 'a' is.

If you know what type you want to use for 'a' you can specify it with
a type annotation, like this:

  pipe = f1 . (f2 :: String -> Int)

-Brent