TE813 — Cannot infer a type argument for an effect operation

Why this fires¶

A generic effect's type parameter couldn't be resolved at a perform site. When an effect declares a type parameter (effect State<T> { fn get() -> T ... }), Zolo infers T from the argument types at the perform call, or from a with State<ConcreteType> pin on the enclosing function — but if neither gives it a concrete type, the operation is genuinely ambiguous.

effect State<T> {
    fn get() -> T
    fn set(v: T)
}

fn run() with State {
    let _ = perform State::get()
    //              ^^^^^^^^^^^^ error[TE813]: cannot infer type argument `T` for
    //                           `State<T>`; annotate explicitly (e.g.
    //                           `perform State<T>::get(...)` or annotate the
    //                           surrounding expression's type)
}

State::get() takes no arguments to unify T against, and the enclosing with State doesn't pin a concrete type either.

Fix it¶

1. Pin the type in the `with` clause¶

fn run() with State<int> -> int {
    return perform State::get()   // ok — T = int from the with-clause pin
}

2. Let an argument fix it¶

fn run() with State {
    perform State::set(99)        // ok — T = int, unified from the argument
}

Cannot infer a type argument for an effect operation

Why this fires¶

Fix it¶

1. Pin the type in the `with` clause¶

2. Let an argument fix it¶

See also¶

See also

Cannot infer a type argument for an effect operation

Why this fires¶

Fix it¶

1. Pin the type in the with clause¶

2. Let an argument fix it¶

See also¶

See also

1. Pin the type in the `with` clause¶