prevents files from being evaluated and stored as values multiple
times. For instance, evaluation of the ‘system’ attribute in NixOS
causes ‘nixpkgs/pkgs/lib/lists.nix’ to be evaluated 2019 times.
Caching gives a modest speedup and a decent memory footprint
reduction (e.g., from 1.44s to 1.28s, and from 81 MiB to 59 MiB with
GC_INITIAL_HEAP_SIZE=100000 on my system).
brackets, e.g.
import <nixpkgs/pkgs/lib>
are resolved by looking them up relative to the elements listed in
the search path. This allows us to get rid of hacks like
import "${builtins.getEnv "NIXPKGS_ALL"}/pkgs/lib"
The search path can be specified through the ‘-I’ command-line flag
and through the colon-separated ‘NIX_PATH’ environment variable,
e.g.,
$ nix-build -I /etc/nixos ...
If a file is not found in the search path, an error message is
lazily thrown.
write ‘attrs ? a.b’ to test whether ‘attrs’ has an attribute ‘a’
containing an attribute ‘b’. This is more convenient than ‘attrs ?
a && attrs.a ? b’.
Slight change in the semantics: it's no longer an error if the
left-hand side of ‘?’ is not an attribute set. In that case it just
returns false. So, ‘null ? foo’ no longer throws an error.
little RAM. Even if the memory isn't actually used, it can cause
problems with the overcommit heuristics in the kernel. So use a VM
space of 25% of RAM, up to 384 MB.
tree). This saves a lot of memory. The vector should be sorted so
that names can be looked up using binary search, but this is not the
case yet. (Surprisingly, looking up attributes using linear search
doesn't have a big impact on performance.)
Memory consumption for
$ nix-instantiate /etc/nixos/nixos/tests -A bittorrent.test --readonly-mode
on x86_64-linux with GC enabled is now 185 MiB (compared to 946
MiB on the trunk).
improves GC effectiveness a bit more (because a live value doesn't
keep other values in the environment plus the parent environments
alive), and removes the need for copy nodes.
a pointer to a Value, rather than the Value directly. This improves
the effectiveness of garbage collection a lot: if the Value is
stored inside the set directly, then any live pointer to the Value
causes all other attributes in the set to be live as well.
errors with position info.
* For all positions, use the position of the first character of the
first token, rather than the last character of the first token plus
one.
values. This improves sharing and gives another speed up.
Evaluation of the NixOS system attribute is now almost 7 times
faster than the old evaluator.
use site, allowing environments to be stores as vectors of values
rather than maps. This should speed up evaluation and reduce the
number of allocations.
precedence, i.e. `with {x=1;}; with {x=2;}; x' evaluates to 2'.
This has a simpler implementation and seems more natural. There
doesn't seem to be any code in Nixpkgs or NixOS that relies on the
old behaviour.
efficiently. The symbol table ensures that there is only one copy
of each symbol, thus allowing symbols to be compared efficiently
using a pointer equality test.
then the blackhole has to be removed to ensure that repeated
evaluation of the same value gives an assertion failure again rather
than an "infinite recursion" error.
that there are some places in Nixpkgs (php_configurable /
composableDerivation, it seems) that call `derivation' with
incorrect arguments (namely, the `name' attribute missing) but get
away with it because of laziness.
* Removed exprToString and stringToExpr because there is no ATerm
representation to work on anymore (and exposing the internals of the
evaluator like this is not a good idea anyway).
allowed. So `name1@name2', `{attrs1}@{attrs2}' and so on are now no
longer legal. This is no big loss because they were not useful
anyway.
This also changes the output of builtins.toXML for @-patterns
slightly.
broken, but now the evaluator checks for it to prevent Nix
expressions from relying on undefined behaviour. Equality tests are
implemented using a shallow pointer equality test between ATerms.
However, because attribute sets are lazy and contain position
information, this can give false positives. For instance,
previously
let y = {x = 1;}; in y == y
evaluated to true, while the equivalent expression
{x = 1;} == {x = 1;}
evaluated to false. So disallow these tests for now. (Eventually
we may want to implement deep equality tests for attribute sets,
like lib.eqStrict.)
* Idem: disallow comparisons between functions.
* Implemented deep comparisons of lists. This had the same problem as
attribute sets - the elements in the list weren't evaluated. For
instance,
["xy"] == [("x" + "y")]
evaluated to false. Now it works properly.
(which means it can only be defined via "inherit"), otherwise we get
scoping bugs, since __overrides can't be recursive (or at least, it
would be hard).
