Using an accumulator, "iterating"...

[Glynn Clements]

Artie Gold wrote:

> > 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?
> > 
> Sure.
> Three comments:
> 
> 1) You don't need (or want) the `do' -- that's used for dealing with 
> monads.
> 
> 2) The function's signature indicates a return type of `Maybe Int', yet 
> you're trying to return an Int.

Maybe *is* a monad:

	instance Monad Maybe where
	    Just x  >>= k = k x
	    Nothing >>= k = Nothing
	    return        = Just
	    fail s        = Nothing

Having said that, treating it as such doesn't really have any benefit
here.

-- 
Glynn Clements <glynn.clements@virgin.net>