[Haskell-cafe] Monad strictness

21 Nov 2005


      In the following, why does testA work and testB diverge?
Where is the strictness coming from?

Thanks,
Yitz

module Test where

import Control.Monad.State
import Control.Monad.Identity

repeatM :: Monad m => m a -> m [a]
repeatM = sequence . repeat

testA =
  take 5 $
  flip evalState [1..10] $ repeatM $ do
    x <- gets head
    modify tail
    return x

testB =
  take 5 $
  runIdentity $
  flip evalStateT [1..10] $ repeatM $ do
    x <- gets head
    modify tail
    return x

[Haskell-cafe] Monad strictness

Yitzchak Gale