Functions and commands

Let's look again at our first example script.

It has a command, introduced by the heading cmd, and a function, introduced by the heading def. These are two very different things.

A command does things: it moves data in and out of the service: in this case getting data from the keyboard and displaying it. You would also use commands to interact with the file system, a database, the system clock, etc. Commands don't return values: they succeed or fail.

A function computes things. It returns a result dependent on the arguments it was called on, and this is all. It has no effect on the outside world, and indeed doesn't know that there is an outside world.

Commands can call functions and other commands: functions can call functions but can't call commands.

This distinction is fundamental to Pipefish. It means that your code must end up as an imperative shell of interaction with the outside world, and a functional core of pure functions expressing business logic.