#3263: Warnings for monadic values not used
-----------------------------+----------------------------------------------
Reporter: NeilMitchell | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.10.2 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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I would like two warnings. The first {{{-fwarn-unused-monad-bind}}}:
{{{
do doesFileExist "foo" ; return 1
}}}
{{{doesFileExist}}} returns a value of {{{IO Bool}}}, not {{{IO ()}}}, and
does not bind it's result. This warning should be applied to both {{{do}}}
notation and {{{>>}}} and should fire on all values which aren't {{{m
()}}}.
The second {{{-fwarn-wrong-bind}}}:
{{{
do return (doesFileExist "foo") ; return 1
}}}
{{{doesFileExist}}} returns a value of type {{{IO (IO a)}}}, which is not
bound. This warning should fire on all {{{m (m a)}}} values and also in
{{{>>}}}.
The names of the flags are intended to be placeholders for a sensibly
chosen name. I would make {{{-fwarn-wrong-bind}}} on as a default warning,
and both turned on under {{{-Wall}}}. I'm not sure anyone has ever written
the second construction without it being a serious error. In the case of
{{{IO}}} it's really easy to miss that some side effects are no longer
happening after a refactoring. I've just made this very mistake in fairly
public way.
It seems that problems such as {{{mapM}}} space leaks are common, and that
it is not widely known that such constructions without a named binding are
always wrong. I've had to explain the problem to fairly experienced
Haskell programmers.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3263>
GHC <http://www.haskell.org/ghc/>
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