[Haskell-cafe] Monads, do and strictness

[David Barbour]

On Sat, Jan 21, 2012 at 10:08 AM, Roman Cheplyaka <roma at ro-che.info> wrote:

> * David Barbour <dmbarbour at gmail.com> [2012-01-21 10:01:00-0800]
> > As noted, IO is not strict in the value x, only in the operation that
> > generates x. However, should you desire strictness in a generic way, it
> > would be trivial to model a transformer monad to provide it.
>
> Again, that wouldn't be a monad transformer, strictly speaking, because
> "monads" it produces violate the left identity law.
>

It meets the left identity law in the same sense as the Eval monad from
Control.Strategies.

http://hackage.haskell.org/packages/archive/parallel/3.1.0.1/doc/html/src/Control-Parallel-Strategies.html#Eval

That is, so long as values at each step can be evaluated to WHNF, it
remains true that `return x >>= f` = f x.

I did mess up the def of >>=. I think it should be:
  (StrictT op) >>= f = StrictT (op >>= \ x -> x `seq` runStrictT (f x))

But I'm not interested enough to actually pull out an interpreter and
test...

Regards,

Dave
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20120121/31708ce3/attachment.htm>