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#322 in Parser implementations

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CircleCI docs.rs

prometheus-parser parses arbitrary Prometheus expressions into a syntax tree appropriate for syntax checking, linting, and general static analysis. It does not depend on a Prometheus server and works entirely offline.

This library is implemented in pure Rust (using pest) with no knowledge of the underlying Prometheus server implementation. It's been validated against the public Prometheus syntax documentation and various publicly available alert collections (e.g. kubernetes-mixin).

Note that this crate doesn't try to evaluate Prometheus expressions; consider using promtool's unit testing for this purpose.

Use cases

This library can be used to implement custom linting and static analysis tools, for example:

  • Offline syntax checking with useful error messages (thanks to pest)
  • Automatic code formatting
  • Various AST inspections:
    • Ensuring expressions don't drop required labels for e.g. alert routing (namespace, service, etc)
    • Ensuring expressions don't make common mistakes that can cause spurious alerts, e.g. division by zero with counters


  • It doesn't try to evaluate expressions at all
  • It doesn't validate function calls or argument types, so foo(bar, baz, qux) is considered at least syntactically valid. Each function argument still needs to parse to a real expression, though.
    • The return_value() utility will raise soft errors on certain type errors (via ReturnKind::Unknown)
    • Note that Prometheus performs some extra query validation based on runtime data and will return errors on e.g. label mismatches that we can't catch.
  • It properly parses most obscure elements of Prometheus syntax, however it's not entirely lossless; equivalent expressions (e.g. sum by (a) (foo{}) and sum (foo) by (a)) will be evaluated identically, so expect minor output differences if you use this to implement a formatter.
  • It probably doesn't parse entirely the same as the real Prometheus parser, though care was taken to match it as closely as possible. That is, this parser may consider expressions valid that Prometheus won't, or vice versa. Please consider filing bugs for any differences.

Return value prediction

The library has a utility for predicting the return types of expressions based only on the syntax tree. This may be useful for rough static analysis.

  • Expression::return_value() recursively computes an expected return type (scalar, instant vector, range vector, etc) along with a set of label operations. If for some reason this fails, ReturnKind::Unknown is returned with a message explaining why.

  • ReturnValue::passthrough(input_labels: &[&str]) can then be used predict which output labels should be present for a given query. It accepts a list of initial label names assumed to be present on the metrics (e.g. expected metrics like namespace) and returns a set of expected output labels.

    Note that additional labels may be returned: if expressions explicitly select for labels, they'll be returned as well; an empty list of initial labels can be used to see what extra labels an expression will query for.

  • ReturnValue::drops(label: &str) determines which AST node, if any, dropped the given input label.



This can be used to dump the (slightly reduced) syntax tree for a given expression.

Try cargo run --example parse 'foo':

$ cargo run --example parse 'count_values
  by(service) ("config_hash", alertmanager_config_hash{job="alertmanager-main",namespace="monitoring"})
  / on(service) group_left() label_replace(max by(name, job, namespace, controller)
  "service", "alertmanager-$1", "name", "(.*)") !=

Operator {
    kind: NotEqual,
    lhs: Operator {
        kind: Divide,
        lhs: Function {
            name: "count_values",
            args: [
                # ... snip ...
            aggregation: Some(
                Aggregation {
                    op: By,
                    labels: [
        rhs: Function {
            name: "label_replace",
            args: [
                # ... snip ...
            aggregation: None,
        matching: Some(
            Matching {
                op: On,
                labels: [
                group: Some(
                    MatchingGroup {
                        op: Left,
                        labels: [],
    rhs: 1.0,
    matching: None,

Note that Expression has a Debug impl that somewhat reduces boilerplate output. It makes debug output more readable, but the displayed object structure won't exactly match the in-memory version. Specifically, if constructing your own AST objects, use the .wrap() utility functions present on most Expression subtypes to convert them to an Expression.


Parses the input expression and reformats it using the default Display impl.

$ cargo run --example reformat 'sum by(bar)(foo)'
sum(foo) by (bar)


Displays the raw ReturnValue computed for a given expression.

$ cargo run --example return_value 'foo(sum(foo))'
ReturnValue {
    kind: InstantVector,
    label_ops: [
        LabelSetOpTuple {
            op: Clear,
            expression: Function {
                name: "sum",
                # ... snip ...
            span: Some(
                Span {
                    start: 4,
                    end: 12,


Shows where a label is dropped in an expression, if at all.

$ cargo run --example label_drop 'foo(sum(foo))' namespace
label 'namespace' is dropped:


parent expression: sum(foo)

If an expression is invalid and has an unknown return type, a message will be printed:

$ cargo run --example label_drop 'foo(sum(foo)) / bar[5m:]' namespace
note: expression return type is unknown, result may be inaccurate
  reason:     rhs return type (RangeVector) is not valid in an operator
  expression: foo(sum(foo)) / bar[5m:]

label 'namespace' is not dropped


Shows the expected output labels for a given expression. Arguments after the expression are labels that exist on all input metrics.

Additional labels may be returned that were not present in the input list; these were inferred based on labels referenced in the expression.

$ cargo run --example label_passthrough 'sum by(bar)(foo)' baz


Bug reports, feature requests, and pull requests are welcome! Be sure to read though the code of conduct for some pointers to get started.

Note that - as mentioned in the code of conduct - code contributions must indicate that you accept the Developer Certificate of Origin, essentially asserting you have the necessary rights to submit the code you're contributing under the project's license (MIT). If you agree, simply pass -s to git commit:

git commit -s [...]

... and Git will automatically append the required Signed-off-by: ... to the end of your commit message.


~74K SLoC