# sim-lib-numbers-func
In one line: It turns a function itself into a value you can name, pass around, and call whenever you like.
## What it gives you
A function, the rule that turns inputs into an output, becomes a thing you can hold in your hand here. You can define one, store it, hand it to another part of your work, and call it later with real inputs to get a result. The rule underneath can be a symbolic formula or a built-in operation, but from the outside it is simply a callable value. You can also ask for its slope, the direction and rate at which its output changes. That makes functions as easy to work with as ordinary numbers.
## Why you will be glad
- Functions become reusable values you can store and pass along like any data.
- The same handle serves whether the rule is a formula or a built-in operation.
- Asking a function for its rate of change is built right in.
## Where it fits
This is the function domain of the SIM number stack. It gives the constellation a common way to carry rules around as data, and it is what the numeric methods reach for when they need something to sample, integrate, or advance step by step.