RX0063

Error message

%s is read inside a nested fn but belongs to the fn around it — a closure (%s)

In the message, %s marks a value that the engine fills in when it raises the error.

The story

You wrote a function inside another function, and the inner one reached out and grabbed a value from the outer one. In many languages that is called a closure, and it works quietly. Rush refuses it, and the reason is a principle: hidden state capture is hidden coupling. The inner function would carry a piece of the outer call's state without anyone ever passing it in. Reading the code, you could not tell what the inner function depends on. Notice that the name is not missing. It is bound, in the enclosing function. So the engine does not say it locates nothing. It says what it really is: a closure. The inner function may use its own parameters, its own values, and the module's names. Nothing else. Example: an outer function holds a rate, and an inner helper multiplies by that rate. Instead, give the helper a second parameter, and pass the rate in when you call it. Now the dependency is visible on the page, and anyone can read it.

Text from the Rush engine's error catalogue.