Expressions, Produced Values, and Blocks

Dewy is expression-based. A literal, call, conditional, block, or loop can participate in a larger expression when it produces a value.

let answer = 42
let larger = answer + 1
let label = if larger >? 40 "large" else "small"

Values, void, and never

Most expressions produce a value. Some operations perform useful work without producing one; their type is void.

Declarations, ordinary assignments, and printl are common void expressions:

let count = 0       # void
count += 1          # void
printl"ready"       # void

never describes a path that cannot complete normally, such as an exit operation. It is not another spelling of void.

none is different again: it is a real value that can be stored in an optional type. Optional Values and Narrowing develops that distinction.

Suppressing a Value

An attached semicolon evaluates an expression but suppresses what it would otherwise produce:

let selected = [
    load_primary();
    load_secondary()
]

Both calls run, but the array collects only the value from load_secondary().

The semicolon is attached to the expression it suppresses. It is not general statement-ending punctuation. An unattached semicolon is reserved for selecting another array dimension.

Blocks

{} forms a scoped block. Expressions inside run from top to bottom, and the block expresses their non-void results:

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

The declaration is void, so the block produces only pi * diameter. diameter belongs to the child scope and is not visible afterward.

Parentheses also group expressions, but do not create a child lexical scope:

let result = (1 + 2) * 3

A block can express several values when its surrounding context knows how to collect them:

let digits = [{ 1 2 3 }]

Loops use the same rule, which is why an array-producing loop needs no separate comprehension syntax. Loop Capture falls out for free.

Comments

# begins a line comment. #{ ... }# is a nestable block comment:

# one line
#{
    an outer comment
    #{ with a nested comment }#
}#

Comment markers inside strings are ordinary string contents.

The Reference specifies evaluation behavior and operator grouping precisely.