E0020 — Effect/purity violation

A function annotated pure (or called from one) invokes a side-effecting operation.

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What triggers it

A pure function’s body (transitively) calls something with observable side effects — I/O, mutable global state, or a builtin tagged as effectful. The effect checker walks the call graph from each pure function and flags the first offending call.

Minimal example

pure fn log_and_add(int a, int b) -> int {
    println("adding");
    return a + b;
}

Output:

scratch.rz:2:5: error[E0020]: effect violation: `println` is not pure
    println("adding");
    ^^^^^^^^^^^^^^^^^^

Fix

Remove the side-effecting call from the pure function, or drop the pure annotation if the effect is intentional.

fn log_and_add(int a, int b) -> int {
    println("adding");
    return a + b;
}

Source

Emitted from the effect-polymorphism checker in resilient/src/effects.rs.