A function is a little machine. Give it a value, and it gives you a new value.

In Nix, a colon : separates the parameter, the local name that receives the input, from what comes back:

Right now the result says «lambda». The machine exists, but we haven’t put anything into it yet.

Put an argument after the function. That is the value fed into its parameter. No commas, and no brackets needed.

The value gets transformed and the whole expression turns into our result!

mynumber is a custom parameter name. It only exists inside the function.

Usually you name a function with let, so you can call it more than once.

double stays a simple multiplication function. The set can reuse it for both money and joy.

Functions are also easy to chain. Use each one to transform the last value, then add a few more and see where it ends up.

Now for the piece of syntax you are going to type more often than any other.

${...} is interpolation. It evaluates whatever is inside and places the result into a string or an attribute name.

Interpolation becomes especially handy inside functions:

One input runs out of usefulness quickly. Write two parameter names to feed a function two values:

Read add 20 22 from left to right: give it 20, then 22. Underneath, Nix does this one input at a time. The nice part is that calls stay pleasantly short.

• input: result creates a function.
• function value calls one.
• first: second: result handles inputs one at a time.
• ${...} drops a calculated value into a string.
• Use a let binding to make a function reusable.