On 2023-06-07 00:37, Bartłomiej Wójcik wrote: > 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?
We can think of Make as a language which has a run-time (rules are matched against the state of the system environment, and build recipes are executed) and a compile-time (Makefiles are read, and manipulated, in order to instantiate all the rules.) All variable manipulations and expansions happen in this compile time. Once make rules are executing, variables (mostly) do not change, except for special variables like $@ (dynamically taking on the value of the target of an executing recipe) and variables set up by the target-specific assignment feature. GNU Make's manual uses the word "evaluate" for all compile-time calculations: assignments and expansions of $(...) syntax. For instance $(shell command ...) is said to evaluate to the output of the command. If VAR contains "a" (or nothing) and VAR := b is processed, then VAR contains b. That is evaluation also. In computer science, evaluation primarily refers to the reduction of a term to its value; however in some imperative languages (C, most Lisp dialects and also Make) side effects like assignment or I/O are understood to be part of evaluation. What is not called evaluation in Make is the processing of the rules against the state of the filesystem to determine what targets need updating, and the execution of the recipes. The term "execution" is used throughout the manual to refer to that processing, as well as the dispatch of external commands (including in $(shell ...)). In a two places, the manual uses the wording "execution of foreach", and "execution of let", which may be accidental slips; those uses could be replaced with "evaluation". The processing of an entire Makefile is referred to as execution in Appendix C, in reference to making a Makefile executable using the #! (hash bang) mechanism.
