File: make.info,  Node: Immediate Assignment,  Next: Conditional Assignment,  Prev: Simple Assignment,  Up: Flavors

6.2.3 Immediately Expanded Variable Assignment
----------------------------------------------

Another form of assignment allows for immediate expansion, but unlike
simple assignment the resulting variable is recursive: it will be
re-expanded again on every use.  In order to avoid unexpected results,
after the value is immediately expanded it will automatically be quoted:
all instances of '$' in the value after expansion will be converted into
'$$'.  This type of assignment uses the ':::=' operator.  For example,

     var = first
     OUT :::= $(var)
     var = second

results in the 'OUT' variable containing the text 'first', while here:

     var = one$$two
     OUT :::= $(var)
     var = three$$four

results in the 'OUT' variable containing the text 'one$$two'.  The value
is expanded when the variable is assigned, so the result is the
expansion of the first value of 'var', 'one$two'; then the value is
re-escaped before the assignment is complete giving the final result of
'one$$two'.

   The variable 'OUT' is thereafter considered a recursive variable, so
it will be re-expanded when it is used.

   This seems functionally equivalent to the ':=' / '::=' operators, but
there are a few differences:

   First, after assignment the variable is a normal recursive variable;
when you append to it with '+=' the value on the right-hand side is not
expanded immediately.  If you prefer the '+=' operator to expand the
right-hand side immediately you should use the ':=' / '::=' assignment
instead.

   Second, these variables are slightly less efficient than simply
expanded variables since they do need to be re-expanded when they are
used, rather than merely copied.  However since all variable references
are escaped this expansion simply un-escapes the value, it won't expand
any variables or run any functions.

   Here is another example:

     var = one$$two
     OUT :::= $(var)
     OUT += $(var)
     var = three$$four

   After this, the value of 'OUT' is the text 'one$$two $(var)'.  When
this variable is used it will be expanded and the result will be
'one$two three$four'.

   This style of assignment is equivalent to the traditional BSD 'make'
':=' operator; as you can see it works slightly differently than the GNU
'make' ':=' operator.  The ':::=' operator is added to the POSIX
specification in Issue 8 to provide portability.