[Haskell-cafe] Printing the function result

[Brent Yorgey]

On 7/25/07, Alexteslin <alexteslin at yahoo.co.uk> wrote:
>
>
> Hi,
>
> I am going through examples from the textbook and trying them out but some
> don't work.
> For example:
>
> addNum :: Int -> (Int -> Int)
> addNum n = addN
>         where
>         addN m = n+m
>
> This error message i am getting:
>
> ERROR - Cannot find "show" function for:
> *** Expression : addNum 4
> *** Of type    : Int -> Int


Hi,

Printing functions is in general not possible, since given a function f
there is no way to "get at" its implementation.*  Functions are like black
boxes.  The only way to observe a function is to observe its effect on
various inputs.  So you can try (addNum 4) 6,  (addNum 4) 0, and so on to
see what the function (addNum 4) does to various Int values.

cheers,
-Brent

*(If there were it would lead to all sorts of fun
non-referential-transparency...)
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