Data.List has `subsequences`, calculating all subsequences of a list, but it doesn't provide a function to check whether a list is a subsequence of another list. `isSubsequenceOf` would go into the "Predicates" section (http://hackage.haskell.org/package/base-4.7.0.1/docs/Data-List.html#g:12) which already contains: * isPrefixOf (dual of inits) * isSuffixOf (dual of tails) * isInfixOf With this proposal, we would add * isSubsequenceOf (dual of subsequences) Suggested implementation: -- | `isSubsequenceOf a b`: Checks if a is a subsequence of b. isSubsequenceOf :: (Eq a) => [a] -> [a] -> Bool isSubsequenceOf [] _ = True isSubsequenceOf _ [] = False isSubsequenceOf a@(x:a') (y:b) | x == y = isSubsequenceOf a' b | otherwise = isSubsequenceOf a b Niklas