dependencyClosure { ... searchPath = [ ../foo ../bar ]; ... }
* Primop `dirOf' to return the directory part of a path (e.g., dirOf
/a/b/c == /a/b).
* Primop `relativise' (according to Webster that's a real word!) that
given paths A and B returns a string representing path B relative
path to A; e.g., relativise /a/b/c a/b/x/y => "../x/y".
determination (e.g., finding the header files dependencies of a C
file) in Nix low-level builds automatically.
For instance, in the function `compileC' in make/lib/default.nix, we
find the header file dependencies of C file `main' as follows:
localIncludes =
dependencyClosure {
scanner = file:
import (findIncludes {
inherit file;
});
startSet = [main];
};
The function works by "growing" the set of dependencies, starting
with the set `startSet', and calling the function `scanner' for each
file to get its dependencies (which should yield a list of strings
representing relative paths). For instance, when `scanner' is
called on a file `foo.c' that includes the line
#include "../bar/fnord.h"
then `scanner' should yield ["../bar/fnord.h"]. This list of
dependencies is absolutised relative to the including file and added
to the set of dependencies. The process continues until no more
dependencies are found (hence its a closure).
`dependencyClosure' yields a list that contains in alternation a
dependency, and its relative path to the directory of the start
file, e.g.,
[ /bla/bla/foo.c
"foo.c"
/bla/bar/fnord.h
"../bar/fnord.h"
]
These relative paths are necessary for the builder that compiles
foo.c to reconstruct the relative directory structure expected by
foo.c.
The advantage of `dependencyClosure' over the old approach (using
the impure `__currentTime') is that it's completely pure, and more
efficient because it only rescans for dependencies (i.e., by
building the derivations yielded by `scanner') if sources have
actually changed. The old approach rescanned every time.
(closed(closed(closed(...)))) since this reduces performance by
producing bigger terms and killing caching (which incidentally also
prevents useful infinite recursion detection).
