gleam-lang/gleam · error

missing guard

Error message

missing guard

What it means

Error "missing guard" thrown in gleam-lang/gleam.

Source

Thrown at compiler-core/src/javascript/decision.rs:902

    }

    fn decision_guard(
        &mut self,
        arena: &'doc DocumentArena<'a, 'doc>,
        guard: usize,
        if_true: &'a Body,
        if_false: &'a Decision,
    ) -> CaseBody<'a, 'doc> {
        let DecisionKind::Case { clauses } = &self.kind else {
            unreachable!("Guards cannot appear in let assert decision trees")
        };

        let guard = clauses
            .get(guard)
            .expect("invalid clause index")
            .guard
            .as_ref()
            .expect("missing guard");

        // Before generating the if-else condition we want to generate all the
        // assignments that will be needed by the guard condition so we can rest
        // assured they are in scope and the guard check can use those.
        let guard_variables = guard.referenced_variables();
        let (check_bindings, if_true_bindings): (Vec<_>, Vec<_>) = if_true
            .bindings
            .iter()
            .partition(|(variable, _)| guard_variables.contains(variable));

        let (check_bindings, check, if_true) = self.inside_new_scope(|this| {
            // check_bindings and if_true generation have to be in this scope so that pattern-bound
            // variables used in guards don't leak into other case branches (if_false).
            let check_bindings = this.variables.bindings_ref_doc(arena, &check_bindings);
            let check = this.variables.expression_generator.guard(arena, guard);
            // All the other bindings that are not needed by the guard check will
            // end up directly in the body of the if clause.
            let if_true_bindings = this.variables.bindings_ref_doc(arena, &if_true_bindings);

View on GitHub (pinned to 7e623aa83d)

When it happens

Trigger: Thrown at compiler-core/src/javascript/decision.rs:902 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/2c393e85a126dc0a. Report an issue: GitHub.