Re: Basic Lexing Question

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From	Kaz Kylheku <480-992-1380@kylheku.com>
Newsgroups	comp.compilers
Subject	Re: Basic Lexing Question
Date	Fri, 1 Jul 2022 04:44:08 -0000 (UTC)
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On 2022-06-29, gah4 <gah4@u.washington.edu> wrote:
> On Wednesday, June 29, 2022 at 2:02:08 PM UTC-7, nob...@gmail.com wrote:
>> The following line is from a makefile accepted by gmake:
>
>> onefile: $(AVAR)
>
>> I'm wondering what the ramification are of lexing what's on the right of the
>> colon as a single string and then breaking it apart later, as opposed to
>> returning a more detailed sequence of tokens, such as DOLLAR LPAREN NAME
>> RPAREN.
>
> I suspect that the question is more complicated than it looks.
>
> Well, first, you might look at the gmake manual, and especially here:
>
> https://www.gnu.org/software/make/manual/html_node/Flavors.html#Flavors
>
> Often in interpreted languages, and also in languages that use a preprocessor,
> you have to consider that things might be parsed more than once.
>
> As well as I know it, in processing that line gmake searches the line for $,
> without (mostly) looking at the rest of the line.  (Even more, I am not sure
> about string constants.)  So variables are replaced, and then the line
> is executed.  Except when it isn't.
>
> It seems that in variable assignment:
>
> bvar = $AVAR
>
> the variable isn't expanded yet, but $AVAR is the value of bvar.
> Then, later, when there is a $bvar, and $AVAR is substituted,
> and then the value of AVAR is substituted.

I believe that = versus := variables can all be handled  at the semantic
level, after an abstract parse.

  bvar = $(AVAR)

is like a macro. When we call $(bvar), it must expand to $(AVAR), which
then expands to the current value of AVAR. That does not  mean we have
to scan any tokens any more; bar can exist in a translated form.

Whereas

  bvar := $(AVAR)

can produce exactly the same representation for the right hand side,
but then evaluate it immediately and capture the resulting string into
bvar.

The one Gmake feature which, I suspect, *must* re-scan the code at
the character level is $(eval ...).  Because, look what you can do:

  DOLLAR := $$
  LPAREN := (
  RPAREN := )

  CODE := $(DOLLAR)$(LPAREN)info CC is $(DOLLAR)$(LPAREN)CC$(RPAREN)$(RPAREN)

  $(eval $(CODE))

  .PHONY: all
  all:

Output:

   CC is cc

Other than eval, I don't suspect anything else requires rescanning;
and note that $(CODE) without eval will not rescan!

So this will not work without eval either:

   $(shell echo foo.o: foo.c)

but this will produce a dependency rule:

   $(eval $(shell echo foo.o: foo.c))

Make can be given a bona-fide "waterfall model of compiling" treatment. :)

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Basic Lexing Question Jon Forrest <nobozo@gmail.com> - 2022-06-29 10:11 -0700
  Re: Basic Lexing Question gah4 <gah4@u.washington.edu> - 2022-06-29 16:27 -0700
    Re: Basic Lexing Question Kaz Kylheku <480-992-1380@kylheku.com> - 2022-07-01 04:44 +0000
  Re: Basic Lexing Question Johann Klammer <klammerj@a1.net> - 2022-06-30 02:38 +0200

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