Scala Interpreter

The Program class is the entry point of the interpreter. The process of executing code goes through the following steps:

• Lexer: scans the input into tokes
• Parser: uses an operator-precedence parser to construct the AST
• Transformers:
  • ClosureTransformer: adds closures to function definitions
  • ShadowReferenceTransformer: renames variables to prevent shadowing
  • LiteralOptimizerTransformer: rewrites expressions who's leaves are all literals to one single literal
• Runtime: executes the code with a treewalker

Values and types

There are four runtime types in this language: Number, Bool, Null and Function. Values are loosely typed and will be converted when needed:

value	Number	Bool	Function
0	self	false	_ -> self
1	self	true	_ -> self
Number	self	error	_ -> self
true	1	self	_ -> self
false	0	self	_ -> self
null	error	error	error
Function	error	error	self

Bools are constructed by the true and false keyword. A null value can be created with null. Numbers are created via literal values, e.g. 1 or 42

Variables

Variables are dynamicly typed, have lexial scoping and can shadow other variables. A variable is delecared with let name := expression;, where the expression will be strictly evaluated. Variables can be updated with name := expression;. Both variable declaration and assignment are statements, but a statement can be converted into an expression by wrapping it in a block expression: let a := { b := 42; };. The value of a here will be null

Expressions

Algebraic:

3 / 2
1 + 2 * 3
3 - 9

Comparison:

1 = 1
1 != 2
1 < 2
2 > 1

Boolean:

1 = 1 | 2 = 4
1 = 1 & 2 = 4
!1

Parentheses:

(1 + 2) * 3

Functions

A function is declared with a single parameter name and an arrow. They have their own lexial scoping when the ClosureTransformer is being used.

a -> a + 2
a -> b -> a + b

A function is called by providing its formal parameter between parentheses:

(a -> a + 2)(1)
(a -> b -> a + b)(1)(2)

Codeblocks:

Codeblocks can be used as expressions. A codeblock can contain both statements and expressions. The value the last statement evaluates to is what the entire codeblock evaluates to.

{ let a := 1 + 2; print a; 2; }

Conditional

if a < b then 1 else 2

Statements

Statements must always and with a semicolon.

Declaration:

All variables are lexial scoped inside codeblocks.

let a := 1;
let b := 1 + 2 * 3;
let c := {
    let a := 1;
    let b := 2;
    a + b;
};

Assinging:

a := 42;

Expression statements:

All expressions can be used as a statement.

Print

The result of any expression can be printed in the console with print:

print 3 / 2;
print 1 + 2 * 3;
print 3 - 9;

While

while a < 5 do {
    print a;
};