Re: State monads don't respect the monad laws in Haskell

Hal Daume <hdaume@ISI.EDU> writes:

[seq is] useful for:

debug :: Show a => a -> a debug x = unsafePerformIO (hPutStrLn stderr (show x)) `seq` x

(Presumably "trace" is defined similarly)

One may ask the question: what is seq useful for not in conjunction with unsafePerformIO, other than efficiency. That, I don't know the answer to.

Of course, this can be defined without seq:

debug :: Show a => a -> a debug x = unsafePerformIO (hPutStrLn stderr (show x) >> return x)

-- Alastair Reid Reid Consulting (UK) Ltd