Re: [Haskell-cafe] Stupid question #852: Strict monad

1 Jan 2009


      On Thu, 2009-01-01 at 17:03 -0500, David Menendez wrote:
...
On Thu, Jan 1, 2009 at 4:30 PM, Jonathan Cast  wrote:
...
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.
Sure, that works.

jcc