c4ac2a164a
instead of `derivation' triggered a huge slowdown in the Nix
expression evaluator. Total execution time of `nix-env -qa' went up
by a factor of 60 or so.
This scalability problem was caused by expressions such as
(x: y: ... x ...) a b
where `a' is a large term (say, the one in
`all-packages-generic.nix'). Then the first beta-reduction would
produce
(y: ... a ...) b
by substituting `a' for `x'. The second beta-reduction would then
substitute `b' for `y' into the body `... a ...', which is a large
term due to `a', and thus causes a large traversal to be performed
by substitute() in the second reduction. This is however entirely
redundant, since `a' cannot contain free variables (since we never
substitute below a weak head normal form).
The solution is to wrap substituted terms into a `Closed'
constructor, i.e.,
subst(subs, Var(x)) = Closed(e) iff subs[x] = e
have substitution not descent into closed terms,
subst(subs, Closed(x)) = Closed(x)
and otherwise ignore them for evaluation,
eval(Closed(x)) = eval(x).
* Fix a typo that caused incorrect substitutions to be performed in
simple lambdas, e.g., `(x: x: x) a' would reduce to `(x: a)'.
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| corepkgs | ||
| doc | ||
| externals | ||
| scripts | ||
| src | ||
| testpkgs | ||
| AUTHORS | ||
| COPYING | ||
| ChangeLog | ||
| INSTALL | ||
| Makefile.am | ||
| NEWS | ||
| README | ||
| configure.ac | ||
| nix.spec.in | ||
| substitute.mk | ||
README
*** Nix *** For installation and usage instructions, please read the manual, which can be found in docs/manual/manual.html, and additionally at the Nix website at <http://www.cs.uu.nl/groups/ST/Trace/NixDeploymentSystem>.