Hi, I think I'm trying to lift 'Data.List.intersperse' (to applicative or a monad) in such a way that its (first) argument is recomputed each time it is used. I'm hoping that there's a reusable, elegant or abstract, approach for this that I'm unaware of. If that isn't clear, I'm using QuickCheck to generate a "sentence" of a random number of random words, each word separated by a random number of spaces. Importantly, there should be no connection between the number of spaces separating the first and second word, and the number of spaces separating the second and third word, etc. I have code which works (run 'workingExample'), but it's not very elegant---I ended up implementing the 'myIntersperse' function manually. I had tried to write the code by fmap-ing Data.List.intersperse (see 'badExample'), but doing that naïvely has a major problem. With that approach, the number of spaces between each word is correctly random between sentences, but is incorrectly constant within each generated sentence. If anybody knows a trick that I'm missing, that would be great. Thanks. {-# LANGUAGE ScopedTypeVariables #-} import Control.Applicative import Control.Monad import Data.List (intersperse) import Test.QuickCheck -- | Generate a string consisting of one or more space character. spaces :: Gen String spaces = elements [" ", " "] -- Generate a (nonsensical) word. word :: Gen String word = elements ["foo", "bar", "baz", "bert"] workingExample, badExample :: IO () workingExample = sample $ myIntersperse spaces (listOf word) badExample = sample $ intersperse <$> spaces <*> listOf word -- Like a lifted version of 'Data.List.intersperse'. The interspersed -- seperator is generated each time the separator appears, as opposed to -- just once for the whole list. myIntersperse :: Gen a -> Gen [a] -> Gen [a] myIntersperse genSep genList = myIntersperse' genSep =<< genList where myIntersperse' :: forall a . Gen a -> [a] -> Gen [a] myIntersperse' genSep [] = return [] myIntersperse' genSep xs = do let listElementWithSep :: Gen [(a, a)] listElementWithSep = zipWithM (\el sep -> pure (el, sep)) xs =<< (sequence . repeat) genSep init <$> tupleListToList <$> listElementWithSep -- | Removes the tuple structure from a list, preserving the -- inner elements and their order. tupleListToList :: [(a, a)] -> [a] tupleListToList = concat . map (\(x, y) -> [x, y]) -- Iain