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feat(tvix/eval): implement attrset update (//) operator

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The underlying implementation does a few tricks based on which pair of
attrset representations is encountered.

Particularly the effect of short-circuiting the empty cases might be
relevant in nixpkgs/NixOS, due to the use of lib.optionalAttrs.

Change-Id: I22b978b1c69af12926489a71087c6a6219c012f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6140
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
This commit is contained in:
Vincent Ambo 2022-08-10 21:01:15 +03:00 • committed by tazjin
parent fa2d250d1a
commit 058e77bab2
13 changed files with 87 additions and 1 deletions
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55
tvix/eval/src/value/attrs.rs
View file

@ -35,7 +35,7 @@ impl Display for NixAttrs {
}
NixAttrs::Map(map) => {
for (name, value) in map {
for (name, value) in map.iter() {
f.write_fmt(format_args!("{} = {}; ", name.ident_str(), value))?;
}
}
@ -54,6 +54,59 @@ impl PartialEq for NixAttrs {
}
impl NixAttrs {
// Update one attribute set with the values of the other.
pub fn update(&self, other: &Self) -> Self {
match (self, other) {
// Short-circuit on some optimal cases:
(NixAttrs::Empty, NixAttrs::Empty) => NixAttrs::Empty,
(NixAttrs::Empty, _) => other.clone(),
(_, NixAttrs::Empty) => self.clone(),
(NixAttrs::KV { .. }, NixAttrs::KV { .. }) => other.clone(),
// Slightly more advanced, but still optimised updates
(NixAttrs::Map(m), NixAttrs::KV { name, value }) => {
let mut m = m.clone();
m.insert(NixString::NAME, name.clone());
m.insert(NixString::VALUE, value.clone());
NixAttrs::Map(m)
}
(NixAttrs::KV { name, value }, NixAttrs::Map(m)) => {
let mut m = m.clone();
match m.entry(NixString::NAME) {
std::collections::btree_map::Entry::Vacant(e) => {
e.insert(name.clone());
}
std::collections::btree_map::Entry::Occupied(_) => {
/* name from `m` has precedence */
}
};
match m.entry(NixString::VALUE) {
std::collections::btree_map::Entry::Vacant(e) => {
e.insert(value.clone());
}
std::collections::btree_map::Entry::Occupied(_) => {
/* value from `m` has precedence */
}
};
NixAttrs::Map(m)
}
// Plain merge of maps.
(NixAttrs::Map(m1), NixAttrs::Map(m2)) => {
let mut m1 = m1.clone();
let mut m2 = m2.clone();
m1.append(&mut m2);
NixAttrs::Map(m1)
}
}
}
/// Retrieve reference to a mutable map inside of an attrs,
/// optionally changing the representation if required.
fn map_mut(&mut self) -> &mut BTreeMap<NixString, Value> {