# XO0701: effect performed outside the declared set

`pub` functions, interface methods, function types, and any function with a `uses` clause are checked against their declared effects; no clause means pure. Performing an effect outside that set, directly or through a call chain, a closure, an interface implementation, or a Go call (effect `go`), is an error, and the diagnostic shows the chain that performs it. Contract clauses (`requires`, `ensures`, `invariant`) must be pure. Declare the effect, or pass the work in from a caller that has it. See core 6.2.

## Example

```xo
pub fn now(c: Clock) -> Time { c.now() }
```

## Fix

```xo
pub fn now(c: Clock) -> Time uses clock { c.now() }
```

Run `xo explain XO0701` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).