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

Excerpts from Edward Z. Yang's message of Tue Dec 08 17:45:19 -0500 2009:

Excerpts from Torsten Otto's message of Tue Dec 08 17:34:30 -0500 2009:

...

testlist 1 = []:[] testlist n = []:(testlist n-1)

Works for me on GHC 6.10.4. What compiler are you using?

Ah, it compiles, but refuses to run. Please ignore me. :-) Edward