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

[Thomas Hartman]

The commented-out signature is incorrect.

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

2008/12/17 Thomas Hartman <tphyahoo at gmail.com>:
> 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)
>