On Fri, Apr 9, 2010 at 10:20 AM, Neil Brown <nc...@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': http://hackage.haskell.org/trac/ghc/ticket/3292 I don't think it's made it into a stable release yet. -- gwern _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
