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
[EMAIL PROTECTED] 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:[EMAIL PROTECTED]
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