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