Make in Prolog 2: Hashes

Kovacsics Robert

Part two of Make in Prolog. This is extending the make implementation with using hashes of inputs instead of modification time for out-of-date detection.

Path Patterns

But first, a discussion on matching on paths. In the previous implementation I used a representation of paths that has some flags in it. Let me show the pathology by studying a few simple paths.

  1. The simplest is main.cpp, which doesn’t work in the build system because we use the pattern Dirs/File so you have to use '.'/main.cpp.
  2. The path_segments_atom translates into a left-associative (a/b)/c which is contrary to expectations and actual the filesystem hierarchy. (Which is a/(b/c) to be clear). This makes matching on filenames easy but makes adding a root directory (or checking the root directory) inconvenient.

The first issue is going away here as we will put the sources into the src directory (and use build for build files).

For now let’s just add a helper to get the root component of the path – I wanted to point the issues out but addressing it is out of scope for now.

make.pl

rootdir(A/B, Root, Rest/B) :- rootdir(A, Root, Rest), !.
rootdir(A/B, A, B).

This never mentions a path without / because it is meant to fail on that, and retry for the last clause to return the root. The cut is there to prevent backtracking and returning multiple directories for root.

Hashes

The overall strategy is that if we have build rules

deps(Base:elf, [Base:obj]).
deps(Base:obj, [Base:cpp]).
build(Path:elf, Deps) :- system(gcc, ['-o', Path:elf | Deps]).
build(Dest:obj, [Src:cpp | Rest]) :-
    system('g++', ['-c', '-o', Dest:obj, Src:cpp | Rest]).

then make(src/main:elf) will

  1. deduce it needs to build/main-Hash1:elf;
  2. which needs build/main-Hash2:obj;
  3. likewise build/main-Hash3:cpp, which it can grab from src/main:cpp and use the hash for the contents of the file;
  4. then it can build build/main-Hash2:obj using build/main-Hash3:cpp, and using the input paths (not the input contents – already captured by their hash1);
  5. likewise build/main-Hash1:elf from build/main-Hash2:obj;
  6. and finally copy build/main-Hash1:elf to src/main:elf.

The code for this at https://git.kovirobi.uk/kovirobi/prolog-make/src/branch/hash on branch hash this time.

  1. This depends on reproducible builds, not the case if you use e.g. __DATE__ or __TIME__ without SOURCE_DATE_EPOCH, see the -Wdate-time option of GCC. ↩︎