Hello, Hirundo

You need two things to use Hirundo: the hirundo binary and your favourite text editor. Write a file, run it. There is no project to configure and no build step to wait through.

Installing

Get Hirundo from your platform’s package manager:

$ brew install hirundo        # macOS
$ apt install hirundo         # Debian & friends
$ winget install hirundo      # Windows

For the programs in this chapter, the version you just installed will do. Hirundo will also handle a project’s version requirement when you encounter one later.

Your first program

Make a file called hello.hir:

let name = "world"
print("hello, \(name)")

hello, world

The panel below the code shows the result. Run the file yourself:

$ hirundo hello.hir

That’s the whole loop: write a file, run it. Everything else in this book builds on it.

Two things happened in those two lines. let name = "world" bound a value to a name, and you didn’t have to say the value was text; Hirundo could see that for itself. Then "hello, \(name)" spliced the value into the middle of a string. Try a friend’s name, or the nearest houseplant’s. The interpolation will keep up.

[!note] When we embed a program, the panel below it shows the expected output or, for a program that cannot run, the expected diagnostic.

Your first oops

Two numbers can look compatible and still have different types. Change the program to do some arithmetic:

let price = 2.5
let items = 2
print(price * items)

error: 3:13 cannot multiply a Double by an Int

Hirundo stopped before running the program. 3:13 means line 3, column 13: the multiplication. price is a decimal number (Double), while items is a whole number (Int). Hirundo points to the mismatch rather than choosing a conversion behind your back. Chapter 2 shows how to make that choice explicitly.

Hirundo reads the file before running it. When it objects, it points.

Trying things out

You don’t need a file for everything. Run hirundo with nothing after it and you get a prompt:

1> let x = 1
x: Int = 1
2> x * 7
7
3> print("hi")
hi

Type a line and the REPL tells you what it means. Bind a name and it echoes the type Hirundo inferred (x: Int = 1); type an expression and it shows the value. Try changing the 7. Keep a prompt open as you read if you want to test an idea without making another file.

(Ctrl-D to leave.)

Where you are

You can run a program from a file, read a type error while the program is still just text, and experiment at a prompt. Chapter 2 picks up price and items where we left them.