On Thu, Jan 1, 2009 at 4:30 PM, Jonathan Cast
On Thu, 2009-01-01 at 13:44 -0700, Luke Palmer wrote:
In my reply I missed the important consideration of the strictness of (>>=), irrsepective of the values. While you can not force values to be strict in a monad without breaking a law, (>>=) is "up for grabs",
Is it? By the second monad law, (>>= return) is required to be strict. return must not be strict, as observed above. Are there monads which satisfy both laws, but have undefined >>= f /= undefined, for some f? I suspect (although I don't seem to have the source on my computer atm) that Control.Monad.State.{Lazy,Strict} both cheat on the second monad law anyway, though...
How about the Identity monad?
ghci> (return undefined :: Identity Char) `seq` ()
*** Exception: Prelude.undefined
ghci> runIdentity $ undefined >>= \_ -> return ()
()
"return" is strict and (>>=) is non-strict.
--
Dave Menendez