[Haskell-cafe] Re: monadic versus aplicative style. These two snips seem to do the same thing yet one has a bug.

It was pointed out to me that there are other differences besides monadic versus applicative, and now I'm thinking I misdiagnosed the bug after all. sorry for the spam. 2008/12/17 Thomas Hartman :

The commented-out signature is incorrect.

Of course, the two functions have the same type sig.

2008/12/17 Thomas Hartman :

...

Hi,

I rewrote a piece of code that used applicative to use instead monadic style, basically because I wanted to adapt it for my own purposes but hadn't wrapped my head around applicative yet.

Unfortunately my rewrite had a bug, which I'm still not completely clear on. (I am guessing it's a laziness mixed with IO issue.)

I discarded my own version and am using the original now of course, but I'm curious if anyone has anything to say on the difference between these two bits of code and, in general if there is any way to "think" about the difference between monadic and applicative.

For what it's worth, the code comes from HSTringTemplate (on hackage). The bug is described in

http://groups.google.com/group/HAppS/browse_thread/thread/70d4b1fbe8a4c7ac

Basically, in happs, this bit of code caused an unrelated handler to not fully pages correctly, where there was more than one image to load. It took forever to diagnose and track this down. Ugh.

Thanks, thomas.

-- incorrect behavior directoryGroup' :: (FilePath -> FilePath -> IO (StringTemplate a)) -> FilePath -> IO (STGroup a) directoryGroup' templatereader path = do fs <- return . ( filter ((".st" ==) . takeExtension) ) =<< getDirectoryContents path templates <- mapM (templatereader path) fs stmapping <- return $ zip fs templates return $ groupStringTemplates stmapping

-- correct behavior --directoryGroup2' :: (Stringable a) => FilePath -> IO (STGroup a) directoryGroup'2 templatereader path = groupStringTemplates <$> (fmap <$> zip . (map dropExtension) <*> mapM ( templatereader path ) =<< filter ((".st" ==) . takeExtension) <$> getDirectoryContents path)