Re: [Haskell-cafe] Different semantics in "identical" do statement?

On Fri, Oct 9, 2009 at 4:25 PM, David Menendez wrote:

On Fri, Oct 9, 2009 at 6:47 PM, staafmeister wrote:

...

Daniel Peebles wrote:

...

I vaguely remember on IRC someone pointing out that the Parsec monad broke one of the laws. I think return _|_ >> x === _|_ which could be causing your problem. I may be wrong though.

Confirmed, working in the parsec monad

Prelude Text.Parsec> runP (do {x <- return undefined; return 10}) () "" "" *** Exception: Prelude.undefined

In the IO Monad Prelude Text.Parsec> do {x <- return undefined; return 10} 10

Should be fixed.

It looks like the problem is a strict field in the definition of GenParser (specifically in Reply). From what I can tell, Parsec 3.0.1 should not have this problem.

-- Dave Menendez http://www.eyrie.org/~zednenem/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

It works with Parsec 3.0.1 for me. Alex