gleam-lang/gleam · error

pattern variable used before assignment

Error message

pattern variable used before assignment

What it means

Error "pattern variable used before assignment" thrown in gleam-lang/gleam.

Source

Thrown at compiler-core/src/javascript/decision.rs:2336

        check_assignments
    }

    /// Returns a string representing the value of a pattern variable: it might
    /// be the code needed to obtain such variable (for example accessing a
    /// list item `wibble.head`), or it could be a name this variable was bound
    /// to in the current scope to avoid doing any repeated work!
    ///
    fn get_value(&self, variable: &Variable) -> EcoString {
        // If the pattern variable was already assigned to a variable that is
        // in scope we use that variable name!
        if let Some(name) = self.scoped_variable_names.get(&variable.id) {
            return name.clone();
        }

        // Otherwise we fallback to using its value directly.
        self.variable_values
            .get(&variable.id)
            .expect("pattern variable used before assignment")
            .clone()
    }

    fn get_segment_value(
        &self,
        arena: &'doc DocumentArena<'a, 'doc>,
        segment_name: &EcoString,
    ) -> Option<Document<'a, 'doc>> {
        // If the segment was already assigned to a variable that is in scope
        // we use that variable name!
        if let Some(name) = self.scoped_segment_names.get(segment_name) {
            return Some(name.clone().to_doc(arena));
        }

        // Otherwise we fallback to using its value directly.
        self.segment_values.get(segment_name).cloned()
    }
}

View on GitHub (pinned to 7e623aa83d)

When it happens

Trigger: Thrown at compiler-core/src/javascript/decision.rs:2336 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of gleam-lang/gleam@7e623aa83d (2026-08-17). Data as JSON: /api/errors/78df13106766cc90. Report an issue: GitHub.