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 6.2 The Two Flavors of Variables
 ================================
 
 There are two ways that a variable in GNU `make' can have a value; we
 call them the two "flavors" of variables.  The two flavors are
 distinguished in how they are defined and in what they do when expanded.
 
    The first flavor of variable is a "recursively expanded" variable.
DONTPRINTYET  Variables of this sort are defined by lines using `=' ( Setting
 Variables Setting.) or by the `define' directive (*note Defining
DONTPRINTYET  Variables of this sort are defined by lines using `=' ( Setting
 Variables Setting.) or by the `define' directive ( Defining

 Variables Verbatim Defining.).  The value you specify is installed
 verbatim; if it contains references to other variables, these
 references are expanded whenever this variable is substituted (in the
 course of expanding some other string).  When this happens, it is
 called "recursive expansion".
 
    For example,
 
      foo = $(bar)
      bar = $(ugh)
      ugh = Huh?
 
      all:;echo $(foo)
 
 will echo `Huh?': `$(foo)' expands to `$(bar)' which expands to
 `$(ugh)' which finally expands to `Huh?'.
 
    This flavor of variable is the only sort supported by other versions
 of `make'.  It has its advantages and its disadvantages.  An advantage
 (most would say) is that:
 
      CFLAGS = $(include_dirs) -O
      include_dirs = -Ifoo -Ibar
 
 will do what was intended: when `CFLAGS' is expanded in a command, it
 will expand to `-Ifoo -Ibar -O'.  A major disadvantage is that you
 cannot append something on the end of a variable, as in
 
      CFLAGS = $(CFLAGS) -O
 
 because it will cause an infinite loop in the variable expansion.
 (Actually `make' detects the infinite loop and reports an error.)  
 
    Another disadvantage is that any functions ( Functions for
 Transforming Text Functions.)  referenced in the definition will be
 executed every time the variable is expanded.  This makes `make' run
 slower; worse, it causes the `wildcard' and `shell' functions to give
 unpredictable results because you cannot easily control when they are
 called, or even how many times.
 
    To avoid all the problems and inconveniences of recursively expanded
 variables, there is another flavor: simply expanded variables.
 
    "Simply expanded variables" are defined by lines using `:=' (
 Setting Variables Setting.).  The value of a simply expanded variable
 is scanned once and for all, expanding any references to other
 variables and functions, when the variable is defined.  The actual
 value of the simply expanded variable is the result of expanding the
 text that you write.  It does not contain any references to other
 variables; it contains their values _as of the time this variable was
 defined_.  Therefore,
 
      x := foo
      y := $(x) bar
      x := later
 
 is equivalent to
 
      y := foo bar
      x := later
 
    When a simply expanded variable is referenced, its value is
 substituted verbatim.
 
    Here is a somewhat more complicated example, illustrating the use of
 `:=' in conjunction with the `shell' function.  ( The `shell'
 Function Shell Function.)  This example also shows use of the variable
 `MAKELEVEL', which is changed when it is passed down from level to
 level.  ( Communicating Variables to a Sub-`make'
 Variables/Recursion, for information about `MAKELEVEL'.)
 
      ifeq (0,${MAKELEVEL})
      whoami    := $(shell whoami)
      host-type := $(shell arch)
      MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
      endif
 
 An advantage of this use of `:=' is that a typical `descend into a
 directory' command then looks like this:
 
      ${subdirs}:
              ${MAKE} -C $@ all
 
    Simply expanded variables generally make complicated makefile
 programming more predictable because they work like variables in most
 programming languages.  They allow you to redefine a variable using its
 own value (or its value processed in some way by one of the expansion
 functions) and to use the expansion functions much more efficiently
 ( Functions for Transforming Text Functions.).
 
    You can also use them to introduce controlled leading whitespace into
 variable values.  Leading whitespace characters are discarded from your
 input before substitution of variable references and function calls;
 this means you can include leading spaces in a variable value by
 protecting them with variable references, like this:
 
      nullstring :=
      space := $(nullstring) # end of the line
 
 Here the value of the variable `space' is precisely one space.  The
 comment `# end of the line' is included here just for clarity.  Since
 trailing space characters are _not_ stripped from variable values, just
 a space at the end of the line would have the same effect (but be
 rather hard to read).  If you put whitespace at the end of a variable
 value, it is a good idea to put a comment like that at the end of the
 line to make your intent clear.  Conversely, if you do _not_ want any
 whitespace characters at the end of your variable value, you must
 remember not to put a random comment on the end of the line after some
 whitespace, such as this:
 
      dir := /foo/bar    # directory to put the frobs in
 
 Here the value of the variable `dir' is `/foo/bar    ' (with four
 trailing spaces), which was probably not the intention.  (Imagine
 something like `$(dir)/file' with this definition!)
 
    There is another assignment operator for variables, `?='.  This is
 called a conditional variable assignment operator, because it only has
 an effect if the variable is not yet defined.  This statement:
 
      FOO ?= bar
 
 is exactly equivalent to this ( The `origin' Function Origin
 Function.):
 
      ifeq ($(origin FOO), undefined)
        FOO = bar
      endif
 
    Note that a variable set to an empty value is still defined, so `?='
 will not set that variable.
 

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