The 3rd exception is raised when (show m) is computed, since no exception occurred when evaluating (testGet L.empty) itself. When you call "evaluate a", it only forces "one level" of evaluation of a. E.g. if you would run
evaluate (1:error "2":[]) >>= print . head
this would print 1, and not give an error. You can maybe understand this better when you change
(res :: Either SomeException String) <- try $ evaluate (f x)
into
(res :: Either SomeException String) <- try $ return (f x)
Now the 2nd exception won't be raised either: evaluate digged one level into assert, and that causes an exception, but return will not do that, it will just return the unevaluated thunk that wraps the assert computation, and this thunk will get evaluated when you print it You can get the behavior what you expect by using some functions from Control.Parallel.Strategies. Change (res :: Either SomeException String) <- try $ evaluate (f x) into (res :: Either SomeException String) <- try $ evaluate (f x `using` rnf) Now all 3 will throw an exception, as you expected. Why? (a `using` rnf) reduces its argument (a) to "head normal form", which basically means it evaluating all sub-expressions at all levels. So
evaluate (1:error "2":[] `using` rnf) >>= print . head
will now give an error.
Cheers,
Peter Verswyvelen
On Sun, Mar 1, 2009 at 1:48 AM, Bjoern Brandenburg
Ok, so I was able to extract a simpler program with the same symptoms.
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1900#a1900
Apparently the exception is only triggered when m is used in line 31. So I guess that evaluate in line 24 is not causing strict evaluation of the Data.Binary.Get monad, even though it is working for the error and assert tests.
Why is that? Is that expected? If so, what am I doing wrong?
Thanks, Bjoern _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners