Getting Started

Welcome to Beamtalk — a Smalltalk-inspired language that compiles to the BEAM (the Erlang virtual machine). You get Smalltalk's elegant message-passing object model on top of BEAM's battle-tested concurrency and fault-tolerance.

This chapter shows you the basics: the REPL, your first expressions, and how to print output.

Starting the REPL

Run the REPL with:

beamtalk repl

You'll see a prompt:

bt>

Type an expression and press Enter. The REPL evaluates it and prints the result.

Your first expressions

Arithmetic works as you'd expect, with standard math precedence:

3 + 4   // => 7
10 - 3  // => 7
2 * 6   // => 12

Division always returns a float:

10 / 4  // => 2.5

Printing output

Transcript show: prints a string to the console. Transcript show: "foo"; cr prints and adds a newline.

In the REPL you'll mostly see return values directly, but in scripts you'll use Transcript:

Transcript show: "Hello, World!"; cr
Transcript show: "Hello, " ++ "Beamtalk!"; cr

Everything is an object

In Beamtalk, everything is an object — integers, strings, booleans, even classes themselves. You interact with objects by sending them messages.

class is a message that asks an object what class it belongs to:

42 class     // => Integer
"hello" class  // => String
true class   // => True
3.14 class   // => Float

printString is a message that asks an object for its string representation:

42 printString    // => 42
true printString  // => true

Comments

Comments start with // and run to the end of the line. There are no multi-line block comments.

// This is a comment — the line below is code:
1 + 1  // => 2

Exercises

1. Explore object classes. Send the class message to five different values (an integer, a float, a string, a boolean, and nil). What class does each report?

42 class       // => Integer
3.14 class     // => Float
"hello" class  // => String
true class     // => True
nil class      // => UndefinedObject

2. Build a greeting. Using string concatenation (++) and the printString message, build the string "I am 25 years old" from the integer 25.

"I am " ++ 25 printString ++ " years old"    // => "I am 25 years old"

3. Chain unary messages. What does -42 abs class evaluate to? Predict the answer before running it, then verify in the REPL.

-42 abs class    // => Integer

What's next

Chapter 2 covers the built-in types: integers, floats, strings, booleans, nil, symbols, and characters.