Szathmáry László honlapja @ DEIK | Nim2 / Subrange types

See this in the manual: https://nim-lang.org/docs/manual.html#types-subrange-types

"A subrange type is a range of values from an ordinal or floating-point type (the base type). To define a subrange type, one must specify its limiting values -- the lowest and highest value of the type."

Examples:

type
Natural = range[0 .. high(int)] # as defined in `system`
Positive = range[1..high(int)] # as defined in `system`
Negative = range[low(int) .. -1] # I missed it from `system`
#
PositiveFloat = range[0.0..Inf]

var
a: Natural
b: Positive
c: int

a = 5 # OK
#a = -2 # Error
b = 3 # OK
#b = -10 # Error

c = a # OK
c = b # OK

var f: PositiveFloat

f = 3.14 # OK
#f = -2.5 # Error

Range object (1..6) and range types (range[1..6])

In Nim, "range" can mean two different things, and they shouldn't be confused.

type
DiceRoll = range[1..6]
# ^ ^
# A B

When you write "1..6", you create a range object. I wrote about it earlier here. It makes a simple object with two fields that define a closed interval. It's just a description of a range; it only does something useful when passed to something that knows how to work with it.
range is a generic type to construct range types. It defines a new type whose values are restricted to that interval. It exists at compile time as a type constraint.