semantics_rules

Semantic Rules

    These rules place additional constraints on the set of valid Decaf programs besides the constraints
    implied by the grammar. A program that is grammatically well-formed and does not violate any
    of the following rules is called a legal program. A robust compiler will explicitly check each of
    these rules, and will generate an error message describing each violation it is able to find. A robust
    compiler will generate at least one error message for each illegal program, but will generate no
    errors for a legal program.

        1. No identifier is declared twice in the same scope. This includes import identifiers, which
           exist in the global scope.
        2. No identifier is used before it is declared.
        3. The program contains a definition for a method called main that has no parameters and
           returns void (note that since execution starts at method main, any methods defined after
           main will never be executed).
        4. The hint literali in an array declaration must be greater than 0.
        5. The number and types of arguments in a method call (non-imports) must be the same as the
           number and types of the formals, i.e., the signatures must be identical.
        6. If a method call is used as an expression, the method must return a result.
        7. String literals and array variables may not be used as arguments to non-import methods.
           Note: a[5] is not an array variable, it is an array location
        8. A return statement must not have a return value unless it appears in the body of a method
           that is declared to return a value.
        9. The expression in a return statement must have the same type as the declared result type
           of the enclosing method definition.
        10. An hidi used as a hlocationi must name a declared local/global variable or formal parameter.
        11. The identifier in a method statement must be a declared method or import.
        12. For all locations of the form <id>[<expr>]
            (a) <id> must be an array variable, and
            (b) the type of <expr> must be int.
        13. The argument of the len operator must be an array variable, and it must return an int
        14. The hexpri in an if or a while statement must have type bool, as well as the second expression
            of a for statement.
        15. The first hexpri in a ternary conditional expression (?:) must have type bool. Each alternative
            in the expr must be of the same type in which it’s context expects the expression to
            be.
        16. The operands of the unary minus, harith opis, and hrel opis must have type int.
        17. The operands of heq opis must have the same type, either int or bool.
        18. The operands of hcond opis and the operand of logical not (!) must have type bool.
        19. The <location> and the <expr> in an assignment, hlocationi = hexpri, must have the same type.
        20. The <location> and the <expr> in an incrementing/decrementing assignment, hlocationi += hexpri
            and <location> -= <expr>, must be of type int. The same is true of hlocationi for ++ and
            - - .
        21. All break and continue statements must be contained within the body of a for or a while.
        22. All integer literals must be in the range −9223372036854775808 ≤ x ≤ 9223372036854775807
            (64 bits).