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Re: Simulating multiple wildcards in a match using .SECONDEXPANSION?

From: Danny Boelens
Subject: Re: Simulating multiple wildcards in a match using .SECONDEXPANSION?
Date: Mon, 06 Apr 2009 14:15:40 +0200
User-agent: Thunderbird (Windows/20090302)

Michael R. Head wrote:
I'd like to make a rule that generates a output file for each pair in
the cross product of two lists of input files. I have a simple makefile
that does what I want, but it's ugly.
I'm wondering if anyone has any suggestions on improving it. At a
minimum, I'd like to get rid of the foreachs in the targets.

You can do that because you know how you constructed the cross product targets. In your case: if you replace the underscore ('_') with a space (' '), the cross product target falls apart into its components again:

As=a1 a2 a3
Bs=b4 b5 b6
ABs=$(foreach a,$(As),$(foreach b,$(Bs),$(a)_$(b)))

empty :=
space := $(empty) $(empty)

A=$(word 1,$(subst _,$(space),$@))
B=$(word 2,$(subst _,$(space),$@))
A_SE=$$(word 1,$$(subst _,$(space),$$@))
B_SE=$$(word 2,$$(subst _,$(space),$$@))

all: $(ABs)

$(As) $(Bs):


$(ABs): $(A_SE) $(B_SE)
        @echo $@: $^
        @echo A=$A, B=$B

But looking at your original problem description, I think I'd use another approach to solve it. One using a define as 'rule template', because it's a bit easier to read and understand in my opinion - but that might be just me. Something like this (untested):

As=a1 a2 a3
Bs=b4 b5 b6

.PHONY: all

$(As) $(Bs) :

define MyRule
all : $(1)_$(2)
$(1)_$(2) : $(1) $(2)
        @echo $$@ : $$^
        @echo A=$1, B=$2

$(foreach a,$(As),$(foreach b,$(Bs),$(eval $(call MyRule,$(a),$(b)))))

Hope this helps,

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