# XO0415: const initializer performs an effect

A top level `const` may call functions, but its initializer's inferred effect set must be empty: no `fs`, `net`, `clock`, `rand`, `env`, `proc`, `stdio`, or `ffi`. It runs once, at the first read, in whichever task reads it first, so an effect there would happen at a time the program text does not show, and the value would depend on more than the program. Read the input in `main` and pass the value down. See core 3.1 and decision 0332.

## Example

```xo
fn stamp() -> Int uses clock {
  42
}

const START = stamp()
```

## Fix

```xo
fn stamp() -> Int uses clock {
  42
}

fn main(os: Os) {
  let start = stamp()
  os.stdio.println(start.display())
}
```

Run `xo explain XO0415` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).