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

[Luke Palmer]

On Thu, Jan 1, 2009 at 2:30 PM, Jonathan Cast <jonathanccast at fastmail.fm>wrote:

> On Thu, 2009-01-01 at 13:44 -0700, Luke Palmer wrote:
> > On Thu, Jan 1, 2009 at 1:31 PM, David Menendez <dave at zednenem.com>
> > wrote:
> >         2009/1/1 Luke Palmer <lrpalmer at gmail.com>:
> >         >
> >         > 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...


ghci> import Control.Monad.Writer
ghci> head . getDual . execWriter $ undefined >> tell (Dual [42])
42

Luke
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090101/e990837e/attachment.htm