Syntax & Semantics: Variable Declaration Typings

[chharvey]

Add typings to variable declarations.

DeclarationVariable
    ::= "let" "unfixed"? IDENTIFIER ":" Type "=" Expression ";";

Example:

let my_var: int = 42;

Declares the my_var with type int (for Integer).

The following type units may be used:

• the null value
• any Boolean, Integer, or Float value itself
• the bool keyword, which denotes “any Boolean value”
• the int keyword, which denotes “any Integer value”
• the float keyword, which denotes “any Float value”
• the obj keyword, which denotes “any value whatsoever (including the null value)”

Type expressions are syntax productions that combine types using operators.

• The unary expression ‹T›! is shorthand for the type union ‹T› | null.
• The binary expression ‹T› & ‹U› is a type intersection, which denotes values of both types ‹T› and ‹U›.
• The binary expression ‹T› | ‹U› is a type union, which denotes values of either type ‹T› or ‹U›.
• Type expressions may contain grouping symbols, e.g., ‹T› & (‹U› | ‹V›).

(‹T› and ‹U› are metavariables for syntactically allowed type expressions.)