On Fri, Apr 9, 2010 at 6:44 AM, Ivan Lazar Miljenovic
<ivan.miljenovic@gmail.com> 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?
Personally, I find it to be tremendously noisy and unhelpful, and I always edit my .cabal files to turn it off. I think of it as a usability regression.
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
If monadic code is going to return an error, then shouldn't _all_
monadic code do so, and not just those in do blocks?
Secondly, a fair number of packages seem to be disabling this globally;
the packages I know of (from mentions on mailing lists and grepping on
/srv/code/*/*.cabal on code.haskell.org) that have the
-fno-warn-unused-do-bind option being passed to GHC in their .cabal file
include:
* HsOpenCL
* leksah-server
* xmonad (including xmonad-contrib)
* xmobar
* pandoc
My reason for bringing this up is that I'm soon about to release a new
version of my graphviz library, and am debating what to do. Note that
most of these errors are being caused by usage of a monadic-style of
parsing (before anyone tells me I should be using an Applicative style
instead, polyparse doesn't support Applicative, so I can't) and as such
the "return value" is being evaluated anyway. The way I see it, I have
4 options:
1. Do as the error suggests and preface usage of these parser
combinators with "_ <-".
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.
What does the Haskell community think? Is -fwarn-unused-do-bind a
worthwhile warning (and code should be updated so as not to cause it to
find anything to warn about)? Or is it more of a hindrance to be
disabled?
--