Statements and discarded values

An expression statement evaluates an expression without binding or returning its result. The current compiler accepts this form only when the result is () or never.

STMT-001 — A non-unit expression result must be handled explicitly

Status: Confirmed

The current compiler rejects a standalone expression whose type is neither () nor never. To discard such a result, source must say so explicitly with drop; to preserve it, source must bind or return it.

effect fn answer() -> i32 { return 42 }

pub effect fn main() {
  drop run answer()
}

Boundary: Calling an effect function as a standalone statement constructs and then attempts to ignore its lazy Effect value. It does not execute the Effect.

effect fn answer() -> i32 { return 42 }

pub effect fn main() {
  answer()
}

The current compiler reports SEM0087 because the statement produces Effect<i32>.

Diagnostics: A non-unit expression statement reports SEM0087 at the expression and includes the produced type. The error must distinguish constructing and discarding an Effect from executing it; it must not imply that the Effect body ran.

This is a uniform statement rule rather than a borrow-checker requirement. It rejects an ignored i32 even though the value is Copy, and it rejects an affine result before automatic cleanup can silently stand in for authored intent. The same rule prevents a lazy Effect construction from looking like execution. drop is the explicit statement that the result is intentionally unwanted.

Dropping an Effect value does not run it. drop answer() constructs and discards the Effect; drop run answer() executes it and discards its successful result.

Evidence: expression-statement requirements, semantic implementation, elaboration tests.