Imperative Blocks

The begin and end keywords introduce an imperative code block. The code block consists of a sequence of statements.

begin
    statement
    statement
    ...
end

The statements are evaluated sequentially until a return statement (see below) is encountered, or the end of the sequence is reached.

A statement may return a value, or it may be void. When a statement returns a value, then all return statements in the code block must agree on the type of the returned value. It is a runtime error if the statement ends without a return. In a void statement all returns must be empty. The code block is evaluted purely for its side-effect and the return statement is optional.

Assignment

An assignment is of the form

var := expression;

or

(var,...,var) := expression;

For all assigned variables in a code block a mutable place is allocated. When an assignment is evaluated the place for that variable is changed to the value of the expression.

The second form is for destructuring tuples. The expression must evaluate to a tuple and all variables in the assignment get assigned to the corresponding tuple value.

Return

A return statement is of the form

return expression;

or

return;

Evaluation of the surrounding begin end block is halted. For a non-void statement the returned value becomes the value of the block.

While

A while statement is of the form

while expression do
    statements
end

The body is a sequence of statements. It is executed as long the expression is true.

If

An if statement is of the form

if expression then
    statements
else if expression then
    statements
...
else
    statements
end

The else and elseif are optional.

The statements in the first branch with a positive test are evaluated.

Procedures

A statement can be a function application. For example

begin
    foo()
    bar()
end

define foo() =
    begin
        ...
    end;

define bar() =
    begin
        ...
    end;

The functions must have type Void. They are evaluated for the side-effect only.