positions :: Eq a => a -> [a] -> [Int]

positions x xs = [i | (x', i) <- zip xs [0..], x == x']

{-

Hutton, Graham. Programming in Haskell (Kindle Locations 1640-1642). Cambridge University Press. Kindle Edition. 

-}

find :: Eq a => a -> [(a,b)] -> [b]

find k t = [v | (k',v) <- t, k == k']

{-

Hutton, Graham. Programming in Haskell (p. 49). Cambridge University Press. Kindle Edition. 

-}

positions' :: Eq a => a -> [a] -> [Int]

positions' x xs = find x (zip xs [0..])

-------------------------------------------------------------------

Having read the chapter a week ago, I had forgotten that Hutton had previously defined a function "find" for tuples (see above). After trying a search of the Kindle book the solution was trivial.

Trent.