Sequencing of Effects

The semicolon operator is used to sequence effects, such as mutating refs. We've seen semicolon occur previously with printing. Now that we're studying mutability, it's time to treat it formally.

• Syntax: e1; e2

• Dynamic semantics: To evaluate e1; e2,

  • First evaluate e1 to a value v1.

  • Then evaluate e2 to a value v2.

  • Return v2. (v1 is not used at all.)

  • If there are multiple expressions in a sequence, e.g., e1; e2; ...; en, then evaluate each one in order from left to right, returning only vn. Another way to think about this is that semicolon is right associative—for example e1; e2; e3 is the same as e1; (e2; e3)).

• Static semantics: e1; e2 : t if e1 : unit and e2 : t. Similarly, e1; e2; ...; en : t if e1 : unit, e2 : unit, ... (i.e., all expressions except en have type unit), and en : t.

The typing rule for semicolon is designed to prevent programmer mistakes. For example, a programmer who writes 2+3; 7 probably didn't mean to: there's no reason to evaluate 2+3 then throw away the result and instead return 7. The compiler will give you a warning if you violate this particular typing rule.

To get rid of the warning (if you're sure that's what you need to do), there's a function ignore : 'a -> unit in the standard library. Using it, ignore(2+3); 7 will compile without a warning. Of course, you could code up ignore yourself: let ignore _ = ().