[Haskell-cafe] Printing the function result

[Alexteslin]

Thank you guys, i didn't understand the concept of the chapter.


Brent Yorgey wrote:
> 
> 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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> 

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