Generic parameter bound as both type and row variable

Why this fires¶

A function's generic parameter list uses the same name both as a type parameter (e.g. in the return type, -> e) and as a row-variable tail in a with clause ({Fs | e}). Zolo's row-polymorphic effects let a function be generic over "whatever other effects the caller's own function needs" — but a single name can't mean both "a type" and "a set of extra effects" at once.

effect Fs { fn read(p: str) -> str }

fn collision<e>(f: fn() with {Fs | e} -> e) with {| e} -> e {
    //         ^ error[TE811]: generic parameter `e` is bound as both a type
    //           variable and a row variable; rename one
    return f()
}

Here e is used as the return type (-> e) AND as the row-variable tail ({Fs | e}) — ambiguous.

Fix it¶

Give the type parameter and the row variable different names:

fn collision<T, e>(f: fn() with {Fs | e} -> T) with {| e} -> T {
    return f()
}

See also¶

• /docs/algebraic-effects — row-polymorphic effects (with {E | r}).