Using an accumulator, "iterating"...
Sat, 21 Jun 2003 14:33:51 +0200
On Saturday, 2003-06-21, 03:23, CEST, Brett Kelly wrote:
> Hello all,
> I'm trying to write a function that takes a list and a element (same type)
> and returns the index of the first instance of the element in the list.
> like: getindex "brett" 'e' would return 2, etc.
> i'm trying to accomplish this using an accumulator, here's what i've got:
> pyindex :: Eq a => a -> [a] -> Maybe Int
> pyindex c l = pyindex' 0 chr (x:xs)
> where pyindex' count chr (x:xs) = do
> if x == chr
> then return count
> else pyindex' (count + 1) chr xs
> now, i know i've got a syntax problem, because i'm pretty sure my logic is
> correct (or at least MOSTLY correct).
> can anybody see what's wrong with my stuff?
I think, the following code, which uses certain prelude functions, would be
clearer and more elegant:
pyindex :: Eq a => a -> [a] -> Maybe Int
pyindex c l = lookup c (zip l [0 ..])
I would it generally consider better style to not do recursion explicitely but
use the recursion already provided by predefined functions like lookup.