Yitzchak Gale wrote:
In the following, why does testA work and testB diverge?
Where is the strictness coming from?
My guess: from strict pattern matching in (>>=).
The following StateT variant uses lazy (irrefutable) pattern match instead.
Regards,
Roberto Zunino.
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newtype StT s m a = StT { runStT :: s -> m (a,s) }
instance (Monad m) => Monad (StT s m) where
return a = StT $ \s -> return (a, s)
m >>= k = StT $ \s -> do
-- was: (a, s') <- runStT m s
~(a, s') <- runStT m s
runStT (k a) s'
fail str = StT $ \_ -> fail str
stGet :: Monad m => StT s m s
stGet = StT $ \s -> return (s,s)
stPut :: Monad m => s -> StT s m ()
stPut s = StT $ \_ -> return ((),s)
evalStT :: Monad m => StT s m a -> s -> m a
evalStT m s = do (x,_) <- runStT m s ; return x
repeatM :: Monad m => m a -> m [a]
repeatM = sequence . repeat
testC =
take 5 $
runIdentity $
flip evalStT [1..10] $ repeatM $ do
s <- stGet
let x = head s
stPut $ tail s
return x
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