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

[Thomas Hartman]

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 <tphyahoo at gmail.com>:
> 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)
>>
>