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

On Thu, 2009-01-01 at 13:44 -0700, Luke Palmer wrote:

On Thu, Jan 1, 2009 at 1:31 PM, David Menendez wrote: 2009/1/1 Luke Palmer : > > So that's the answer: there already is a Strict monad. And an attempt to > make a lazier one strict just results in breaking the monad laws.

There is at least one transformer that will make a strict monad out of a non-strict monad.

newtype CPS m a = CPS { unCPS :: forall b. (a -> m b) -> m b }

I have heard this called the "codensity monad" (and it appears under that name in category-extras). Good observation.

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... jcc