expr

`qgis_render::expr`

The QGIS expression engine: the same language used for rule-based symbology, labels, and field calculators, available from Rust.

use qgis_render::expr::*;

Parsing and evaluating

let expr = Expression::parse("\"height\" > 50 AND \"type\" = 'residential'")?;

let result = expr.evaluate(&feature, &context)?;
let matched = result.as_bool();

evaluate returns a Value, which converts to Rust types:

Method	Returns
`as_bool()`	`bool`
`as_f64()`	`f64`
`as_i64()`	`i64`
`as_string()`	`String`
`as_geometry()`	`Option<Geometry>`
`is_null()`	`bool`

Supported syntax

Expression::parse("length($geometry)")?;
Expression::parse("concat(\"name\", ' (', \"height\", 'm)')")?;
Expression::parse("CASE WHEN \"pop\" > 1000 THEN 'big' ELSE 'small' END")?;
Expression::parse("color_ramp_interpolate('RdYlGn', \"value\", 0, 100)")?;

Field names are double-quoted, string literals single-quoted. $geometry, $id, and $area refer to the current feature.

Building expressions programmatically

let expr = Expression::gt(
    Expression::field("height"),
    Expression::literal(50),
);

Constructor	Description
`Expression::parse(&str)`	Parse expression text.
`Expression::field(&str)`	A field reference.
`Expression::literal(v)`	A constant.
`Expression::gt` / `lt` / `gte` / `lte` / `eq` / `ne`	Comparisons.
`Expression::and` / `or` / `not`	Boolean logic.
`Expression::function(name, args)`	Any registered QGIS function.

Compiling for repeated evaluation

Parsing is the expensive part. Compile once against a layer’s fields, then evaluate per feature:

let compiled = expr.compile(&layer.fields())?;

for feature in layer.features() {
    if compiled.evaluate_bool(&feature)? {
        // ...
    }
}

Using expressions as filters

Filter::expr wraps an expression so it can be pushed into a FeatureRequest:

use qgis_render::features::{FeatureRequest, Filter};

let request = FeatureRequest::new()
    .filter(Filter::expr("\"height\" > 50 AND \"type\" = 'residential'"));

for feature in layer.features_with(request) {
    // ...
}

Errors

Expression::parse and evaluate return QgisError::ExpressionError { expr, message } with the offending expression and the parser’s diagnostic.