Using an accumulator, "iterating"...

[mike castleman]

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On Fri, Jun 20, 2003 at 06:23:42PM -0700, Brett Kelly wrote:
> now, i know i've got a syntax problem, because i'm pretty sure my logic is
> correct (or at least MOSTLY correct).

Try:

pyindex :: Eq a => a -> [a] -> Maybe Int
pyindex c l = pyindex' 0 l
    where pyindex' _ [] = Nothing
          pyindex' n (x:xs) = if (x == c)
                              then Just n
                              else pyindex' (n+1) xs


then, in ghci:
Pyindex> pyindex 'e' "brett"
Just 2
Pyindex> pyindex 'm' "brett"
Nothing
Pyindex> pyindex 't' "brett"
Just 3

hope this helps.
mike

- -- 
mike castleman / m at mlcastle dot net / http://mlcastle.net / (646) 382-7220
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