[Haskell-cafe] WriterT [w] IO is not lazy in reading [w]

David Menendez dave at zednenem.com
Thu Jan 1 14:16:40 EST 2009

On Thu, Jan 1, 2009 at 5:17 AM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> It might be possible to build a "lazy-ify" monad transformer which
> would make this work.  In particular, I think Oleg's LogicT
> transformer[1] does something of the sort, only applying side effects
> where they are required in order to get "the next result" in a
> nondeterministic computation.

LogicT is continuation-based, so it's strict in the first argument to (>>=).

    observeT $ undefined >>= \_ -> return ()

will throw an exception.

On the other hand, LogicT is non-strict in the second argument to
mplus if the transformed monad is non-strict in (>>=).

    take 4 . runIdentity . observeAllT . msum $ map return [0..]

should return [0,1,2,3].

(My implementation does. I assume the LogicT on Hackage is the same.)

Dave Menendez <dave at zednenem.com>

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