#3263: Warnings for monadic values not used
------------------------------+---------------------------------------------
Reporter: NeilMitchell | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.2
Severity: normal | Resolution:
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Changes (by batterseapower):
* cc: [email protected] (added)
Comment:
I've attached above an implementation of these two flags. The patch
contains:
* Code adding -fwarn-wrong-do-bind and -fwarn-unused-do-bind warning
generation to the compiler
* Documentation for the two new flags
* Changes to all the core libraries and GHC itself to prevent the build
breaking with -Werror
* Tests for the new warnings
I found one particularly suspicious instance of code that the warnings
picked up in GHC:
{{{
; _ <- set_head_fact h a fuel
; return fuel }
}}}
(compiler/cmm/ZipDataflow.hs:784:22:). set_head_fact returns a Fuel, but
it is ignored by the single use site. Can this really be what was
intended?
When modifying all the libraries, I found a number of functions that often
required me to write tedious {{{_ <-}}} bindings to suppress the warnings.
Maybe we should consider providing _ variants of the following:
{{{
forkIO -- Returns a ThreadId, which is hardly ever used
char -- Returns the character input verbatim, noone cares about it
string -- Returns the string input verbatim
evaluate -- Returns the thing on the input. Perhaps we should provide
force :: a -> IO () as an alternative
try -- I saw a use of try discarding the result (i.e. ignoring all
exceptions) in both Cabal and ghc (InteractiveUI.hs)
throwErrnoIfNull -- One occurence
}}}
Internal stuff:
{{{
writeBuffer -- Seen in the encoding code. Return value rarely used.
installHandler -- Used a lot in GHC code, return value rarely (never?)
used
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3263#comment:3>
GHC <http://www.haskell.org/ghc/>
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