[Haskell-cafe] Re: what is inverse of mzero and return?

Am Dienstag, 25. Januar 2005 03:25 schrieb Jan-Willem Maessen:

I feel obliged to point out (because the repeated references to the question are driving me up the wall) that this simple equality holds in every monad:

return 4 >> return 5

=== (definition of >>)

return 4 >>= \_ -> return 5

=== (monad laws)

(\_ -> return 5) 4

=== (beta reduction)

return 5

We don't need to know anything about the semantics, etc. of any other actions the monad might happen to define.

-Jan-Willem Maessen

Thanks, nice to be taken back down to earth. So we have putStrLn "hello" >> mzero === putStrLn "hello" >>= (\_ -> mzero) === (\_ -> mzero) () === mzero indeed. Daniel