Prior to this change, we had a bunch of ad-hoc string manipulation code
scattered around. This made it hard to figure out what data model for
string contexts is.
Now, we still store string contexts most of the time as encoded strings
--- I was wary of the performance implications of changing that --- but
whenever we parse them we do so only through the
`NixStringContextElem::parse` method, which handles all cases. This
creates a data type that is very similar to `DerivedPath` but:
- Represents the funky `=<drvpath>` case as properly distinct from the
others.
- Only encodes a single output, no wildcards and no set, for the
"built" case.
(I would like to deprecate `=<path>`, after which we are in spitting
distance of `DerivedPath` and could maybe get away with fewer types, but
that is another topic for another day.)
This makes the position object used in exceptions abstract, with a
method getSource() to get the source code of the file in which the
error originated. This is needed for lazy trees because source files
don't necessarily exist in the filesystem, and we don't want to make
libutil depend on the InputAccessor type in libfetcher.
When calling `builtins.readFile` on a store path, the references of that
path are currently added to the resulting string's context.
This change makes those references the *possible* context of the string,
but filters them to keep only the references whose hash actually appears
in the string, similarly to what is done for determining the runtime
references of a path.
The documentation for `parseDrvName` does not agree with the implementation when
the derivation name contains a dash which is followed by something that is
neither a letter nor a digit. This commit corrects the documentation to agree
with the implementation.
The current definition of `intersectAttrs` is incorrect:
> Return a set consisting of the attributes in the set e2 that also exist in the
> set e1.
Recall that (Nix manual, section 5.1):
> An attribute set is a collection of name-value-pairs (called attributes)
According to the existing description of `intersectAttrs`, the following should
evaluate to the empty set, since no key-value *pair* (i.e. attribute) exists in
both sets:
```
builtins.intersectAttrs { x=3; } {x="foo";}
```
And yet:
```
nix-repl> builtins.intersectAttrs { x=3; } {x="foo";}
{ x = "foo"; }
```
Clearly the intent here was for the *names* of the resulting attribute set to be
the intersection of the *names* of the two arguments, and for the values of the
resulting attribute set to be the values from the second argument.
This commit corrects the definition, making it match the implementation and intent.
* libexpr: fix builtins.split example
The example was previously indicating that multiple whitespaces would be
collapsed into a single captured whitespace. That isn't true and was
likely a mistake when being documented initially.
* Fix segfault on unitilized list when printing value
Since lists are just chunks of memory the individual elements in the
list might be unitilized when a programming error happens within Nix.
In this case the values are null-initialized (at least with Boehm GC)
and we can avoid a nullptr deref when printing them.
I ran into this issue while ensuring that new expression tests would
show the actual value on an assertion failure.
This is unlikely to cause any runtime performance regressions as
printing values is not really in the hot path (unless the repl is the
primary use case).
* Add operator<< for ValueTypes
* Add libexpr tests
This introduces tests for libexpr that evalulate various trivial Nix
language expressions and primop invocations that should be good smoke
tests wheter or not the implementation is behaving as expected.
