Fernando Rodriguez wrote:
Hi,
I defiend the following function to get the last element of a list:
final [a] = a final (_:t) = final t
and it works as expected. Since I didn't want to have a non exhaustive pattern, I added the following case:
final [] = [] - I consider that the end of an empty list is the empty list final [a] = a final (_:t) = final t
Suddenly, the function stoped working with a rather cryptic (for a newbie at least) error message:
*Temp> final [4,5]
<interactive>:1:9: No instance for (Num [a]) arising from the literal `5' at <interactive>:1:9 Possible fix: add an instance declaration for (Num [a]) In the expr*Temp> ession: 5 In the first argument of `final', namely `[4, 5]' In the expression: final [4, 5]
What have I done so wrong?
You probably want final to have type final :: [a] -> a But the equation final [] = [] conflicts with this. BTW, you might want to have this kind of discussion at beginners@haskell.org instead. See the announcement: http://thread.gmane.org/gmane.comp.lang.haskell.general/16345 Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de