HuwCampbell / haskell-list-builder

Mutable List Builders in Haskell

Haskell List Builder

Extra unsafe sequencing of IO actions from Twan van Laarhoven packaged up, along with unfoldIO and an implementation of Scala's List Buffer. This means we can have tail recursive sequence and traverse specialised for lists of IO actions.

For algorithms which are more easily written in a mutable way we have ListBuilder s a. A ListBuilder s a is like a wrapper around an ST s [a], but with a constant time append as well as prepend.

Basic usage:

import qualified Data.ListBuilder as ListBuilder

palindrome :: [a] -> [a]
palindrome xs = runST $ do
  bldr <- ListBuilder.newBuilder
  for_ xs $ \x -> do
    ListBuilder.prepend x bldr
    ListBuilder.append x bldr
  ListBuilder.unsafeFreeze bldr

Other functions of interest:

traverseIO :: (a -> IO b) -> [a] -> IO [b]
sequenceIO :: [IO a] -> IO [a]
unfoldIO :: IO (Maybe a) -> IO [a]