m (tagged the implementation found in prelude.)
Revision as of 19:54, 30 November 2012
(*) Reverse a list.
reverse :: [a] -> [a] reverse = foldl (flip (:)) 
The standard definition, found in the prelude, is concise, but not very readable. Another way to define reverse is:
reverse :: [a] -> [a] reverse  =  reverse (x:xs) = reverse xs ++ [x]
However this definition is more wasteful than the one in Prelude as it repeatedly reconses the result as it is accumulated. The following variation avoids that, and thus computationally closer to the Prelude version.
reverse :: [a] -> [a] reverse list = reverse' list  where reverse'  reversed = reversed reverse' (x:xs) reversed = reverse' xs (x:reversed)