Re: if expression and match expression

On Fri, Oct 09, 2015 at 03:03:47PM +0200, Gabriel Scherer [email protected] [ocaml_beginners] wrote:
> I personally dis-trust "when" guards (which is why I didn't include
> them in my reply). I have seen people over-use them and get clever
> bugs because of it (a bit of the same way you were surprised with your
> simpler bug), and they also prevent the compiler from being able to
> tell whether a pattern-matching is exhaustive or has superfluous cases
> -- because it does not try to guess the meaning of the boolean test. I
> would use an if-then-else in your situation. That said, it is a matter
> of taste, and many people will like your code as well.
> 
> Another thing I am sensitive to is that having "_" at the toplevel of
> a pattern is bad form in my book. It does not matter much for
> algebraic types that are known to have two cases (lists and options),
> but whenever you are pattern-matching on a type that *you* defined and
> that may get new constructors in the future, using "_" as a pattern
> will prevent the compiler from warning you that this pattern-matching
> need to be adapted (because it is not exhaustive anymore). Thus I try
> to take the habit never to use "_" as a whole pattern (it is often fine
> under a constructor), at any type (otherwise it's easy to slip back).
> The version with "if" doesn't have a "_" as head pattern, so it is better
> in that respect as well.

So you'd recommend something like

let rec has_element2 l e =
match l with
| [] -> false
| h::t -> h = e or has_element2 (List.tl l) e
;;

-- hendrik