[Haskell] ANNOUNCE: generator 0.5.1

Yitzchak Gale gale at sefer.org
Thu Jul 16 11:18:33 EDT 2009

Yair Chuchem wrote:
> A new "generator" package has been uploaded to Hackage.
> It implements an alternative list monad transformer, a list class, and
> related functions.

This, and just about all of the other "similar" types mentioned
in this thread, are all new names for the venerable "ListT Done Right"
that has been around for many years:


The ListT that is (still, unbelievably) in the mtl package
is very broken, and shouldn't be used. It's not even
a monad - it doesn't satisfy the monad laws.

So why make up new names? Just call it ListT.

I agree, ListT is a very nice approach. Perhaps that
is why this wheel has been re-invented so many times,
many more even than what has been brought up in this

Unfortunately, given the way the standard libraries are
currently implemented in GHC, ListT IO cannot be made
exception-safe, so ListT has limited usefulness for real
software. The main problem is the type of the primitives
(un)blockAsyncExceptions# which makes it possible to
block exceptions only in IO itself, not in transformations
of IO. That, in turn, makes it impossible to implement
things like bracket and finally in transformations of IO.

What we need is something like

startBlocking :: IO ()
stopBlocking :: IO ()


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