The expression `with E1; E2' evaluates to E2 with all bindings in
the attribute set E1 substituted. E.g.,
with {x = 123;}; x
evaluates to 123. That is, the attribute set E1 is in scope in E2.
This is particularly useful when importing files containing lots
definitions. E.g., instead of
let {
inherit (import ./foo.nix) a b c d e f;
body = ... a ... f ...;
}
we can now say
with import ./foo.nix;
... a ... f ...
I.e., we don't have to say what variables should be brought into scope.
print a nice backtrace of the stack, rather than vomiting a gigantic
(and useless) aterm on the screen. Example:
error: while evaluating file `.../pkgs/system/test.nix':
while evaluating attribute `subversion' at `.../pkgs/system/all-packages-generic.nix', line 533:
while evaluating function at `.../pkgs/applications/version-management/subversion/default.nix', line 1:
assertion failed at `.../pkgs/applications/version-management/subversion/default.nix', line 13
Since the Nix expression language is lazy, the trace may be
misleading. The purpose is to provide a hint as to the location of
the problem.
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)'.
`bla:' is now no longer parsed as a URL.
* Re-enabled support for the `args' attribute in derivations to
specify command line arguments to the builder, e.g.,
...
builder = /usr/bin/python;
args = ["-c" ./builder.py];
...
Nix. This is to prevent Berkeley DB from becoming wedged.
Unfortunately it is not possible to throw C++ exceptions from a
signal handler. In fact, you can't do much of anything except
change variables of type `volatile sig_atomic_t'. So we set an
interrupt flag in the signal handler and check it at various
strategic locations in the code (by calling checkInterrupt()).
Since this is unlikely to cover all cases (e.g., (semi-)infinite
loops), sometimes SIGTERM may now be required to kill Nix.
Eelco Dolstra