Hi, Thank you for the response. I think that I understand that, but still not catching the part with other things. What is the term *evaluation*, when does it happen (in which phase) and how is it related to parsing, expanding?
Regards, Bartek śr., 7 cze 2023 o 09:08 Kaz Kylheku <k...@kylheku.com> napisał(a): > On 2023-06-06 15:14, Bartłomiej Wójcik wrote: > > Hi, > > > > I would like to ask you for help with clarifying the idea of evaluation > and > > expansion related to makefile. As I understand, expansion refers to > > replacing macro reference by its value and there are two possible ways of > > expansion - *immediate* and *deferred* which depends on the construction > > itself. I also found that in the *immediate* context the expansion is > done > > before evaluation, and for the *deferred* context, it is the opposite. > But > > it is also stated that in the *immediate *context, the expansion is done > > while parsing, which would be contradictory, because as I understand the > > evaluation is the process of parsing and internalizating. > > The terms "eager" and "lazy" could be used. > > # eager/immediate: $(bar) is expanded, combined with abc, and assigned > to foo > foo := $(bar) abc > > # lazy/deferred: foo is assigned the unexpanded right hand side. > foo = $(bar) abc > > # eager/immediate: here, the variable is not in the right hand side > # of a lazy definition's assignment; its value is required > # immediately, just like in the right hand side of := > target: prerequisite $(foo) > build step > > Outside the right side of an assignment, everything is immediate: > when the value of a variable is needed, it is replaced by its > content, which is fully expanded, as necessary. > > Mostly, the syntax determines whether the right hand side of > an assignment is expanded immediately or deferred. Except > that certain kinds of assignments like += rely on whether the > variable was previously the target of a deferred or immediate > assignment. For the purpose of these operators, variables have > to track the style in which they were most recently assigned. > That attribute can optimize expansion. If we know that we > need the value of $(foo), and we know that it was assignned using > IMMEDIATE := IMMEDIATE, then we know we don't have to run an expansion > pass on it; just dump the content where we need it. > > The documentation has a diagram like this: > > IMMEDIATE = DEFERRED > IMMEDIATE ?= DEFERRED > IMMEDIATE := IMMEDIATE > IMMEDIATE ::= IMMEDIATE > IMMEDIATE += DEFERRED or IMMEDIATE > IMMEDIATE != IMMEDIATE > > define IMMEDIATE > DEFERRED > endef > > # ... etc ... not going to reproduce all of it > > Why is there IMMEDIATE on the left hand sides? Because > left hand sides can contain expansions too: computed > variables. Things like: > > OBJS_$(this_file) = foo.o bar.o > > The above diagram tells us that this is IMMEDIATE = DEFERRED > which tells us that OBJS_$(this_file) is expanded immediately > in order to determine the name of the variable which receives > the right hand side body. > >
