Re: Why 'statement may have no effect' lint warning here?

[arnold]

I agree that in this case the warning is wrong. My improved code
no longer issues the warning for this case.

Arnold

Wolfgang Laun <wolfgang.laun@gmail.com> wrote:

> It's not so simple. See below. Is the warning a bug or not? No side effect
> - no effect.
>
> function f(x){
>    a[1] = "hooey"
> }
>
> BEGIN {
> i = 1
> f(i) == 47 ## Warning here
> print a[1]
> }
> -W
>
>
> On Tue, 13 Apr 2021 at 10:58, <arnold@skeeve.com> wrote:
>
> > Hi.
> >
> > Arkadiusz Drabczyk <arkadiusz@drabczyk.org> wrote:
> >
> > > > > The following code produces 'statement may have no effect' when run
> > > > > with --lint:
> > > > >
> > > > > BEGIN {
> > > > >     var = 1
> > > > >     var == 1 && var = 2
> > > > >     print "var ==", var
> > > > > }
> > > >
> > > > I think that it's a bug that it does so.
> > >
> > > ok, so just to make sure - is this an actual bug?
> >
> > Yes. Because:
> >
> > > > The intent for that warning is soemthing like:
> > >
> > > > BEGIN {
> > > >     var = 1
> > > >     var == 2        # no effect
> > > > }
> >
> > > I see why lint would warn me in that case but I still don't understand
> > > that part:
> > >
> > > > See above; if `var == 1' is used standalone as a statement,
> > > > it has no effect -- nothing is changed as result.
> > >
> > > ok, nothing is changed, that is value returned by == is not saved
> > > anywhere but it's an expression, right? == operator returns either 0
> > > or 1.
> >
> > Yes, but because the result is thrown away and nothing done with it,
> > the statement has no effect: no variables are changed, no 'if' or
> > loop statement is affected, etc.
> >
> > > That
> > >
> > >     var = 1
> > >     var == 2 && var = 3
> > >     print "var ==", var
> > >
> > > would print 1 because var == 2 is not true and `var = 3' would not be
> > > executed. So `var == 2' has an effect because it effects the other
> > > side of &&.
> >
> > And that's why I said that the current warning is a bug.
> >
> > > BTW (or maybe not?) the original snippet I posted cannot be reproduced
> > > with Busybox implementation of awk but works with mawk and FreeBSD
> > > awk. So maybe doing things like `var == 2 && var = 3' is not specified
> > > by POSIX and cannot be expected to work?
> >
> > You can expect it to work. It's perfectly fine. It sounds to me like
> > a bug in Busybox awk.
> >
> > > Subject: Re: Why 'statement may have no effect' lint warning here?
> > > To: Arkadiusz Drabczyk <arkadiusz@drabczyk.org>
> > > From: Manuel Collado <mcollado2011@gmail.com>
> > > Date: Tue, 13 Apr 2021 09:45:48 +0200
> > >
> > > [ .... ]
> > >
> > > The lint warning is 'statement may have no effect'. The key part is the
> > > 'may' word. Just a hint that in some cases what the programmer wrote was
> > > not what (s)he wanted.
> > >
> > > A more elaborate example:
> > >
> > > $ cat no-effect.awk
> > > BEGIN {
> > >      var = "abcd"
> > >      sub("a", "x", var) && sub("c", "y", var)
> > >      print "var ==", var
> > >
> > >      var = "abcd"
> > >      sub("e", "x", var) && sub("c", "y", var)
> > >      print "var ==", var
> > > }
> > >
> > > $ gawk --lint -f no-effect.awk
> > > gawk: no-effect.awk:3: warning: statement may have no effect
> > > gawk: no-effect.awk:7: warning: statement may have no effect
> > > var == xbyd
> > > var == abcd
> >
> > I disagree that the warning here is valid. At least one of the
> > sub() calls will execute, so it has an effect.
> >
> > I am working to improve the lint warnings. I will  post a patch soon.
> >
> > Thanks,
> >
> > Arnold
> >
> >