[Haskell-beginners] recursively building lists

[Torsten Otto]

Thank you! I'll go hide in a dark corner...
Torsten

Am 08.12.2009 um 23:43 schrieb Daniel Fischer:

> Am Dienstag 08 Dezember 2009 23:34:30 schrieb Torsten Otto:
>> Hi!
>> 
>> While trying to implement "words", we ran into the question of how to build
>> lists of lists. The trouble boils down to this:
>> 
>> test = []:[]
>> 
>> is no problem, just as
>> 
>> Prelude> []:[]:[]:[]
>> [[],[],[]]
>> 
>> works fine. Now trying to put the two together _is_ a problem:
>> 
>> testlist 1 = []:[]
>> testlist n = []:(testlist n-1)
>> 
>>    No instance for (Num [[a]])
>>      arising from a use of `testlist' at <interactive>:1:0-9
>>    Possible fix: add an instance declaration for (Num [[a]])
>>    In the expression: testlist 2
>>    In the definition of `it': it = testlist 2
>> 
>> Can someone please explain, what is going on here?
> 
> Yes. Function application has higher precedence than aritthmetic operations, so the RHS of 
> testlist n is parsed as
> 
> [] : ( (testlist n) - 1)
> 
> So, by the equation for testlist 1, the compiler knows that
> 
> testlist :: (Num n) => n -> [[a]]
> 
> In the second equation, you try to subtract 1 from a value of type [[a]], that means you 
> need a Num instance for that type.
> 
> What you want is
> 
> testlist n = []:testlist (n-1)
> 
> HTH,
> Daniel