This internal module provides functions used to define the various enumFrom* functions of Enum.

We expect fromEnum to be an ordering homomorphism, that is:

forall a b. Enum a b
succ a == b => fromEnum a < fromEnum b

Note that this homomorphism is most likely not surjective. Note further that one cannot assume:

CANNOT BE ASSUMED !
succ a == b => fromEnum a + 1 == fromEnum b

The succ essor of a given message enum value A that's not maxBound is the enum value B whose fromEnum value is the one immediately after A's fromEnum value. That is, fromEnum determines order, but not distance.

As an example, consider the enum in the test suite:

enum Baz {
    BAZ1 = 1; BAZ2 = 2; BAZ3 = 4; BAZ4 = 6;
    BAZ5 = 7; BAZ6 = 9; BAZ7 = 10; BAZ8 = 12;
}

In this case, succ BAZ2 is BAZ3 despite their fromEnum values differing by 2. Further, [BAZ2, BAZ4 ..] or equivalently messageEnumFromThen BAZ2 BAZ4 is every other enum (i.e. a distance of 2) when taken as a list, i.e. [BAZ2, BAZ4, BAZ6, BAZ8] despite the fromEnum distances being [4, 3, 3].

That said, it is highly unwise to use any of the [a,b ..*] patterns or enumFromThen* functions since adding or removing enums values can cause previously functioning code to fail. I.e. removing BAZ3 in the above example makes the result equivalent fromEnum BAZ2 and the sequence now includes every enum value save BAZ1. This is all despite the fact that BAZ3 was never referenced.