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make: Foreach Function

 
 8.5 The 'foreach' Function
 ==========================
 
 The 'foreach' function is very different from other functions.  It
 causes one piece of text to be used repeatedly, each time with a
 different substitution performed on it.  It resembles the 'for' command
 in the shell 'sh' and the 'foreach' command in the C-shell 'csh'.
 
    The syntax of the 'foreach' function is:
 
      $(foreach VAR,LIST,TEXT)
 
 The first two arguments, VAR and LIST, are expanded before anything else
 is done; note that the last argument, TEXT, is *not* expanded at the
 same time.  Then for each word of the expanded value of LIST, the
 variable named by the expanded value of VAR is set to that word, and
 TEXT is expanded.  Presumably TEXT contains references to that variable,
 so its expansion will be different each time.
 
    The result is that TEXT is expanded as many times as there are
 whitespace-separated words in LIST.  The multiple expansions of TEXT are
 concatenated, with spaces between them, to make the result of 'foreach'.
 
    This simple example sets the variable 'files' to the list of all
 files in the directories in the list 'dirs':
 
      dirs := a b c d
      files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
 
    Here TEXT is '$(wildcard $(dir)/*)'.  The first repetition finds the
 value 'a' for 'dir', so it produces the same result as '$(wildcard
 a/*)'; the second repetition produces the result of '$(wildcard b/*)';
 and the third, that of '$(wildcard c/*)'.
 
    This example has the same result (except for setting 'dirs') as the
 following example:
 
      files := $(wildcard a/* b/* c/* d/*)
 
    When TEXT is complicated, you can improve readability by giving it a
 name, with an additional variable:
 
      find_files = $(wildcard $(dir)/*)
      dirs := a b c d
      files := $(foreach dir,$(dirs),$(find_files))
 
 Here we use the variable 'find_files' this way.  We use plain '=' to
 define a recursively-expanding variable, so that its value contains an
 actual function call to be re-expanded under the control of 'foreach'; a
 simply-expanded variable would not do, since 'wildcard' would be called
 only once at the time of defining 'find_files'.
 
    The 'foreach' function has no permanent effect on the variable VAR;
 its value and flavor after the 'foreach' function call are the same as
 they were beforehand.  The other values which are taken from LIST are in
 effect only temporarily, during the execution of 'foreach'.  The
 variable VAR is a simply-expanded variable during the execution of
 'foreach'.  If VAR was undefined before the 'foreach' function call, it
 is undefined after the call.  The Two Flavors of Variables
 Flavors.
 
    You must take care when using complex variable expressions that
 result in variable names because many strange things are valid variable
 names, but are probably not what you intended.  For example,
 
      files := $(foreach Esta-escrito-en-espanol!,b c ch,$(find_files))
 
 might be useful if the value of 'find_files' references the variable
 whose name is 'Esta-escrito-en-espanol!' (es un nombre bastante largo,
 no?), but it is more likely to be a mistake.
 

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