gnu make 的 call function

這只是節錄 nmake 的 documentation:

The syntax of the call function is:    
$(call variable,param,param,...)
make 執行這道命時，會賦予param...一個暫時的變數名稱 $(1), $(2), etc.
變數 $(0) 則是 variable. 
Then variable is expanded as a make variable in the context of these temporary assignments.  Thus, any reference to $(1) in the value of variable will resolve to the first param in the invocation of call.
If variable is the name of a builtin function, the builtin function is always invoked (even if a make variable by that name also exists).  
examples
 This macro simply reverses its arguments:  
reverse = $(2) $(1)
foo = $(call reverse,a,b)
  
  foo 會是  `b a'.  
 This one is slightly more interesting: it defines a macro to search for the first instance of a program in PATH:  
pathsearch = $(firstword $(wildcard $(addsufix /$(1),$(subst :, ,$(PATH)))))
LS := $(call pathsearch,ls)

Now the variable LS contains /bin/ls or similar.

The call function can be nested. Each recursive invocation gets its own local values for $(1), etc. that mask the values of higher-level call. For example, here is an implementation of a map function:

map = $(foreach a,$(2),$(call $(1),$(a)))

Now you can map a function that normally takes only one argument, such as origin, to multiple values in one step:

o = $(call map,origin,o map MAKE)

and end up with o containing something like `file file default'.

A final caution: be careful when adding whitespace to the arguments to call. As with other functions, any whitespace contained in the second and subsequent arguments is kept; this can cause strange effects. It's generally safest to remove all extraneous whitespace when providing parameters to call.