Expressions and Evaluation

Every executable construct in Dewy is an expression. An expression may produce one value, several values for a surrounding collector, void, or never when it cannot complete.

Produced Values

Literals, value-returning calls, exhaustive conditionals, and value-producing blocks can supply surrounding expressions. Declarations and ordinary assignments produce void.

An attached postfix semicolon evaluates an expression and suppresses the values it would otherwise produce:

operation();

An unattached semicolon is reserved for array-dimension selection and does not act as generic statement punctuation.

Blocks

{} is a scoped block. () groups expressions without introducing a child lexical scope.

A block evaluates its expressions in source order and expresses each non-void result. A context requiring one value rejects a block that can produce an incompatible number of values.

let circumference = {
    let diameter = 2 * radius
    pi * diameter
}

Only the final calculation produces a value because the declaration is void.

Evaluation Order

Within the expression tree established by grouping and precedence, operands and call arguments evaluate from left to right. A construct documented as evaluating an operand once must preserve that behavior even if lowering expands it into several primitive operations.

Boolean short-circuit expressions evaluate only the operands required by their truth rule. Flow alternatives evaluate conditions in order and execute only the selected body.

Assignment

Assignment evaluates its destination place and source, updates the binding or selected field/element, and produces void. Combined assignment loads the old value, applies the selected typed operator, and stores the result while evaluating the destination route only once.

See Operators and Precedence, Bindings, and Values, Copies, and Places.

Place Projection

The prefix @ must begin a place route and selects the location at the end of the complete member-and-index route:

@pair.left
@values[i]
@box.rows[row][column]

The parser groups the prefix before the selectors, but that grouping does not make the root place an independently observable intermediate value. @(pair.left) selects the same final place as @pair.left; Dewy has no pair.@left form.

Function handles extend the same whole-route rule with a grouping boundary between selection or partial evaluation and an ordinary call. See Function Handles.