2006/6/22, Sara Kenedy <[EMAIL PROTECTED]>:
Hello all,
Now I am trying with the function of polymorphic type: This function
returns the Nth element of list with type a. I try it as below.
getNthElem :: Int -> [a] -> Maybe a
getNthElemt _ [] = Nothing
getNthElem 0 _ = Nothing
getNthElem n s
| n > length s = Nothing
| otherwise = Just ((drop (n-1) (take n s))!!0)
>getNthElem 2 ["a","b","c"]
Just "b"
However, I do not satisfy with this function because I want to return
the Nth element of type a, not (Maybe a). For example, I want this
function:
getNthElem :: Int -> [a] -> a
But, I do not know how to define the empty element of type a.
getNthElemt _ [] = ????
getNthElem 0 _ = ????
If you have some ideas about this, please give me some clues. Thanks a lot.
hi,
precisely, you want to return an "a" only when there is one
accordingly to your above code. the only way to handle this without
resorting to [] or Nothing to say there is no such value is to use
error or default value.
infact, ask yourself, what do you want ?
getNthElem 5 ["a","b","c"]
or
getNthElem 0 ["a","b","c"]
or
getNthElem (-1) ["a","b","c"]
do you want
"a"
[]
Nothing
"wrong"
or raise an exception (i.e. you use the "error" function) ?
once you know what you want, you can code it.
note, i think your 'take" is unnecessary here
| otherwise = Just ((drop (n-1) (take n s))!!0)
also
you can use
| otherwise = Just (n!!some_value) -- :)
this is where you see that the function you're trying to write is
really close of
(!!)
cheers,
mt
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
