builtins.genericClosure is a quite powerful (and undocumented) Nix
primop: It repeatedly applies a function to values it produces and
collects them into a list. Additionally individual results can be
identified via a key attribute.
Since genericClosure only ever creates a single list value internally,
we can eliminate a huge performance bottleneck when building a list in a
recursive algorithm: list concatenation. Because Nix needs to copy the
entire chunk of memory used internally to represent the list, building
big lists one element at a time grinds Nix to a halt.
After rewriting decode using genericClosure decoding the LaTeX source
of my 20 page term paper now takes 2s instead of 14min.
Change-Id: I33847e4e7dd95d7f4d78ac83eb0d74a9867bfe80
8615322bc8
