[bug #64339] $(filter) and $(filter-out) interpret "match" in surprising ways

[Eli Zaretskii]

URL:
  <https://savannah.gnu.org/bugs/?64339>

                 Summary: $(filter) and $(filter-out) interpret "match" in
surprising ways
                   Group: make
               Submitter: eliz
               Submitted: Thu 22 Jun 2023 10:07:13 PM IDT
                Severity: 3 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
       Component Version: SCM
        Operating System: Any
           Fixed Release: None
           Triage Status: None


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Follow-up Comments:


-------------------------------------------------------
Date: Thu 22 Jun 2023 10:07:13 PM IDT By: Eli Zaretskii <eliz>
The GNU Make Manual says:

'$(filter-out PATTERN...,TEXT)'
     Returns all whitespace-separated words in TEXT that _do not_ match
     any of the PATTERN words, removing the words that _do_ match one or
     more.  This is the exact opposite of the 'filter' function.

     For example, given:

          objects=main1.o foo.o main2.o bar.o
          mains=main1.o main2.o

     the following generates a list which contains all the object files
     not in 'mains':

          $(filter-out $(mains),$(objects))

Note that the text doesn't really say what it means to "match" in this
context, nor what exactly is PATTERN. The example might make you believe the
"match" means "substring". That is A "matches" B if A is some substring of B.
But this is not the case. Observe:

$ make -f-
ORIG=foo123 bar-xyz baz-yes
EXCLUDE=bar foo
all:
        @echo "$(filter-out $(EXCLUDE),$(ORIG))"
^D
=> foo123 bar-xyz baz-yes

That is, "foo123" does NOT match "foo", and "bar-xyz" does NOT match "bar".
The following does work, though:

$ make -f-
ORIG=./foo ./bar ./baz
EXCLUDE=%bar %foo
all:
        @echo "$(filter-out $(EXCLUDE),$(ORIG))"
^D
=> ./baz

So now we perhaps understand that PATTERN is something that includes the %
wildcard somewhere. This should be explained in the manual, and the manual
should also somehow justify the special case of exact string equality, in
which case the % wildcard is not required.

IOW, having the exact string equality as the only example in this case is
misleading and confusing. I learned this the hard way, having just spent an
hour debugging my Makefile where $(filter-out) didn't do its job, because I
thought "./foo" should match "foo", based on what the manual says.








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