Informal description of the λanguage

Before anything, we should have a clear picture about what we're trying to achieve. It's a good idea to put together a rigorous description of the grammar, but I'm going to keep things more informal in this tutorial, so here is the λanguage by examples:

# this is a comment

println("Hello World!");

println(2 + 3 * 4);

# functions are introduced with `lambda` or `λ`
fib = lambda (n) if n < 2 then n else fib(n - 1) + fib(n - 2);

println(fib(15));

print-range = λ(a, b)             # `λ` is synonym to `lambda`
                if a <= b then {  # `then` here is optional as you can see below
                  print(a);
                  if a + 1 <= b {
                    print(", ");
                    print-range(a + 1, b);
                  } else println("");        # newline
                };
print-range(1, 5);

Note above that identifier names can contain the minus character (print-range). That's a matter of personal taste: I always put spaces around operators, I don't like much camelCaseNames and the dash is nicer than the underscore. The nice thing about writing your own language is that you can do it as you like it. :)

The output is:

Hello World!
14
610
1, 2, 3, 4, 5

The λanguage looks a bit like JavaScript, but it's different. First, there are no statements, only expressions. An expression returns a value and can be used in place of any other expression. Semicolons are required to separate expressions in a “sequence”. The curly brackets, { and }, create such a sequence, and it's itself an expression. Its value is what the last expression evaluates to. The following is a valid program:

a = {
  fib(10);  # has no side-effects, but it's computed anyway
  fib(15)   # the last semicolon can be missing
};
print(a); # prints 610

Functions are introduced with one of the keywords lambda or λ (they are synonyms). After the keyword there must be a (possibly empty) parenthesized list of variable names separated with commas, like in JavaScript — these are the argument names. The function body is a single expression, but it can be a sequence wrapped in {…}. There is no return statement (there are no statements) — the last expression evaluated in a function gives the value to return to its caller.

There is no var. To introduce new variables, you can use what JavaScripters call “IIFE”. Use a lambda, declare variables as arguments. Variables have function scope, and functions are closures — like in JavaScript.

Even if is itself an expression. In JavaScript you'd get that effect with the ternary operator:

a = foo() ? bar() : baz();           // JavaScript
a = if foo() then bar() else baz();  # λanguage

The then keyword is optional when the branch starts with an open bracket ({), as you can see in print-range above. Otherwise it is required. The else keyword is required if the alternative branch is present. Again, then and else take as body a single expression, but you can {group} multiple expressions by using brackets and semicolons. When the else branch is missing and the condition is false, the result of the if expression is false. Speaking of which, false is a keyword which denotes the only falsy value in our λanguage:

if foo() then print("OK");

will print "OK" if and only if the result of foo() is NOT false. There's also a true keyword for completion, but really everything which is not false (in terms of JavaScript's === operator) will be interpreted as true in conditionals (including the number 0 and the empty string "").

Also note above that there is no point to demand parentheses around an if's condition. It's no error if you add them, though, as an open paren starts an expression — but they're just superfluous.

A whole program is parsed as if it were embedded in curly brackets, therefore you need to place a semicolon after each expression. The last expression can be an exception.

Well, that's our tiny λanguage. It's not necessarily a good one. The syntax looks cute, but it has its traps. There are a lot of missing features, like objects or arrays; we don't concentrate on them because they're not essential for our journey. If you understand all this material, you'll be able to implement those easily.

In the next section we'll write a parser for this λanguage.