Catch-all Cases

One thing to beware of when pattern matching against variants is what Real World OCaml calls "catch-all cases". Here's a simple example of what can go wrong. Let's suppose you write this variant and function:

type color = Blue | Red
(* a thousand lines of code in between *)
let string_of_color = function
  | Blue -> "blue"
  | _    -> "red"

Seems fine, right? But then one day you realize there are more colors in the world. You need to represent green. So you go back and add green to your variant:

type color = Blue | Red | Green

But because of the thousand lines of code in between, you forget that string_of_color needs updating. And now, all the sudden, you are red-green color blind:

# string_of_color Green
- : string = "red"

The problem is the catch-all case in the pattern match inside string_of_color: the final case that uses the wildcard pattern to match anything. Such code is not robust against future changes to the variant type.

If, instead, you had originally coded the function as follows, life would be better:

let string_of_color = function
  | Blue -> "blue"
  | Red  -> "red"

Now, when you change color to add the Green constructor, the OCaml type checker will discover and alert you that you haven't yet updated string_of_color to account for the new constructor:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:                              

The moral of the story is: catch-all cases lead to buggy code. Avoid using them.

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