[Haskell-cafe] What is the consensus about -fwarn-unused-do-bind ?

Gwern Branwen gwern0 at gmail.com
Fri Apr 9 11:22:25 EDT 2010

On Fri, Apr 9, 2010 at 10:20 AM, Neil Brown <nccb2 at kent.ac.uk> wrote:
> Ivan Lazar Miljenovic wrote:
>> As of 6.12.1, the new -fwarn-unused-do-bind warning is activated with
>> -Wall.  This is based off a bug report by Neil Mitchell:
>> http://hackage.haskell.org/trac/ghc/ticket/3263 .
>> However, does it make sense for this to be turned on with -Wall?  For
>> starters, why should this warning apply only to do blocks and not to
>> explicit usage of >>, etc.?  That is, the following code (as specified
>> in the above bug report) generates an error:
>>   do doesFileExist "foo"
>>      return 1
>> yet this doesn't:
>>    doesFileExist "foo" >> return 1
> The comments in that bug report actually mention "My patch does not warn on
> uses of >>, only in do-notation, where the situation is more clear cut".  I
> take >> to be an explicit sign that the user wants to ignore the result of
> the first action, whereas in do-notation it may be an accident.  So I think
> it was the right decision.

Relevant link: http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.html

>>  2. Use some function of type "(Monad m) => m a -> m ()" instead of doing
>>     "_ <-".
>>  3. Duplicate the parser combinators in question so that I have one
>>     version that returns a value and another that does the main parser
>>     and then returns (); then use this second combinator in do blocks
>>     where I don't care about the returned value.
>>  4. Put "-fno-warn-unused-do-bind" in the .cabal file.
>> The first two options don't appeal to me as being excessive usage of
>> boilerplate; the third involves too much code duplication.  However, I
>> am loath to just go and disable a warning globally.
> I'd be tempted by number two, but I it's more typing to write "ignore $"
> than "_ <-", so maybe 1 is the best option after all.  I've frequently
> encountered the annoyance of monadic return values -- but to satisfy type
> signatures rather than avoid this warning.  For example, I have a CHP
> parallel operator: (<||>) :: CHP a -> CHP b -> CHP (a,b) and a function
> writeChannel :: Chanout a -> a -> CHP ().  But if you try to write a
> function like:

It's actually going to be named 'void':

I don't think it's made it into a stable release yet.


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