Util.hs

-----------------------------------------------------------------------------
--
-- Module      :  Control.Monad.Util
-- Copyright   :  (c) 2017 Brian W Bush
-- License     :  MIT
--
-- Maintainer  :  Brian W Bush <consult@brianwbush.info>
-- Stability   :  Stable
-- Portability :  Portable
--
-- | Utilities for monads.
--
-----------------------------------------------------------------------------


module Control.Monad.Util (
-- * Iteration
  collectWhile
) where


-- | Collect the results of running an action repeatedly while a condition holds.
collectWhile :: Monad m
             => m a    -- ^ The action to repeat.
             -> m Bool -- ^ The action for the condition that must hold.
             -> m [a]  -- ^ An action collecting the results.
collectWhile next test =
  do
    t <- test
    if t
      then do
             n <- next
             (n :) <$> collectWhile next test
      else return []