Values and Constants

A program often begins with facts rather than storage: a screen has a fixed number of rows, a protocol assigns a byte value to a command and a feature is either enabled or disabled. Nucleus represents those facts with constants.

const rows = 4
const columns = $08
const enabled = true
const stepBack = -1
assert rows * columns = %100000

The initializer determines a scalar constant's type. enabled is Boolean. The other constants are exact integers: the compiler retains their mathematical values and chooses a stored integer type each time they are used. This lets rows fit a u8 use in one place and a u16 use in another.

Five scalar types

Nucleus has four integer types and one Boolean type.

Type	Values
u8	0 through 255
u16	0 through 65,535
i8	-128 through 127
i16	-32,768 through 32,767
boolean	false or true

Unsigned types suit byte values, sizes and addresses within a bounded object. Signed types suit quantities that may cross zero. A Boolean is not an integer: Nucleus does not treat zero as false or one as true.

Decimal literals need no prefix. $ introduces hexadecimal and % introduces binary. A character literal such as 'A' has the byte value 65. These forms all describe integers; the surrounding use chooses a suitable type.

Checked conversion

Nucleus permits only value-preserving implicit conversions: u8 to u16, u8 to i16 and i8 to i16. Other conversions must be visible:

var byteValue as u8 = u8(wordValue)

u8(...), u16(...), i8(...) and i16(...) check the value at run time when it is not already known. A value outside the destination range causes a safety trap; it is never silently truncated. When the compiler knows the value is out of range, it rejects the source instead.

Assertions check the design

assert requires a Boolean constant expression. It emits no target code. The declaration above records that the chosen dimensions multiply to 32. If a later edit changes one constant but not the relationship, compilation stops at the assertion.

The companion program combines all three literal bases, a character value, a negative exact constant and a checked conversion. It finishes with observed equal to 97.

const rows = 4
const columns = $08
const enabled = true
const stepBack = -1
assert rows * columns = %100000

var observed as u16 = 0

sub main()
    if enabled
        observed = u16(rows * columns) + u16('A') + u16(i8(stepBack) + 1)
    end
end

Summary

• Scalar constants infer Boolean or exact-integer values.
• Exact integers adopt a suitable stored integer type at each use.
• Explicit integer conversion checks the mathematical value.
• $ and % introduce hexadecimal and binary literals.
• assert checks a Boolean constant expression without generating code.

The governing rules are in the specification chapters on source text, types and declarations. The checked companion is 02-values.nu.