Llaisdy

Learn you a Haskell describes these two type classes like this:

Ord is for types that have an ordering.
…
Enum members are sequentially ordered types.

I didn’t find this especially clear.

Real World Haskell failed (yet again). It [the printed book] gives no definition of Enum, although it seems to think it does (p. 472).

Mostly for my own benefit, this is what I’ve found:

The GHC documentation describes the two type classes like this (emphasis added):

The Ord class is used for totally ordered datatypes.
…
Class Enum defines operations on sequentially ordered types.

So both classes represent ordered types (both are instances of the Ordering data type). The difference is in the kind of ordering.

A clue is in the methods provided: Ord provides >, <, max and min; Enum provides succ and pred (and, interestingly, nothing similar to max and min).

Finally, the best explanation I found was in Chapter 8: Standard Haskell Classes of A Gentle Introduction to Haskell:

8.2 The Enumeration Class

Class Enum has a set of operations that underlie the syntactic sugar of arithmetic sequences; for example, the arithmetic sequence expression [1,3..] stands for enumFromThen 1 3. We can now see that arithmetic sequence expressions can be used to generate lists of any type that is an instance of Enum. This includes not only most numeric types, but also Char, so that, for instance, [‘a’..’z’] denotes the list of lower-case letters in alphabetical order. Furthermore, user-defined enumerated types like Color can easily be given Enum instance declarations. If so:

[Red..Violet] => [Red, Green, Blue, Indigo, Violet]

Note that such a sequence is arithmetic in the sense that the increment between values is constant, even though the values are not numbers. Most types in Enum can be mapped onto integers; for these, the fromEnum and toEnum convert between Int and a type in Enum.

Note however, that for some reason the Ord type class includes “All Prelude types except IO, (->), and IOError”, so a simple mathematical interpretation won’t do.

Conclusion

So! Actually I’m going to ignore that last caveat.

I think the comfiest way for me to understand these type classes is using the available methods: Ord types are those for which it makes sense to talk about relationships like “greater than” and “less than”; Enum types are those for which it makes sense to talk about relationships like “next” and “previous”. Days of the week would be Enum; Dates would be Ord.