Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You just wrote the name of a function in a place where a value belongs, and the engine said no. This is one of the deepest ideas in rush: an action is not a value. A function is something you do, a value is something you do things to. The engine does not mix them up. So you cannot bind a function to a name, pass it as an argument, store it in a map, or use it as a receiver. Notice that the engine does not say the name is missing, because it is not missing. The name locates a verb, and a verb is called, never held. Example. You wrote f equals dbl, hoping to keep the function around. Instead, call it and keep its answer: f equals dbl applied to a value. And if you want to choose between several actions, write a flow whose arms call each function by name. The message names the function and shows the lawful spelling for this site, so you can copy the shape. Call actions. Hold values. That small discipline keeps scripts honest.
Text from the Rush engine's error catalogue.