Using a variable in pattern matching in Ocaml or F#

Question

I have a function of the form

'a -> ('a * int) list -> int

let rec getValue identifier bindings = 
  match bindings with
  | (identifier, value)::tail -> value
  | (_, _)::tail -> getValue identifier tail
  | [] -> -1

I can tell that identifier is not being bound the way I would like it to and is acting as a new variable within the match expression. How to I get identifier to be what is passed into the function?

Ok! I fixed it with a pattern guard, i.e. | (i, value)::tail when i = indentifier -> value but I find this ugly compared to the way I originally wanted to do it (I'm only using these languages because they are pretty...). Any thoughts?

Answer 1

You can use F# active patterns to create a pattern that will do exactly what you need. F# supports parameterized active patterns that take the value that you're matching, but also take an additional parameter.

Here is a pretty stupid example that fails when the value is zero and otherwise succeeds and returns the addition of the value and the specified parameter:

let (|Test|_|) arg value = 
  if value = 0 then None else Some(value + arg)

You can specify the parameter in pattern matching like this:

match 1 with
| Test 100 res -> res // 'res' will be 101

Now, we can easily define an active pattern that will compare the matched value with the input argument of the active pattern. The active pattern returns unit option, which means that it doesn't bind any new value (in the example above, it returned some value that we assigned to a symbol res):

let (|Equals|_|) arg x = 
  if (arg = x) then Some() else None

let foo x y = 
  match x with
  | Equals y -> "equal"
  | _ -> "not equal"

You can use this as a nested pattern, so you should be able to rewrite your example using the Equals active pattern.

Answer 2

One of the beauties of functional languages is higher order functions. Using those functions we take the recursion out and just focus on what you really want to do. Which is to get the value of the first tuple that matches your identifier otherwise return -1:

let getValue identifier list = 
match List.tryFind (fun (x,y) -> x = identifier) list with
    | None      -> -1
    | Some(x,y) -> y

//val getValue : 'a -> (('a * int) list -> int) when 'a : equality

This paper by Graham Hutton is a great introduction to what you can do with higher order functions.

Answer 3

This is not directly an answer to the question: how to pattern-match the value of a variable. But it's not completely unrelated either.

If you want to see how powerful pattern-matching could be in a ML-like language similar to F# or OCaml, take a look at Moca.

You can also take a look at the code generated by Moca :) (not that there's anything wrong with the compiler doing a lot of things for you in your back. In some cases, it's desirable, even, but many programmers like to feel they know what the operations they are writing will cost).

Answer 4

What you're trying to do is called an equality pattern, and it's not provided by Objective Caml. Objective Caml's patterns are static and purely structural. That is, whether a value matches the pattern depends solely on the value's structure, and in a way that is determined at compile time. For example, (_, _)::tail is a pattern that matches any non-empty list whose head is a pair. (identifier, value)::tail matches exactly the same values; the only difference is that the latter binds two more names identifier and value.

Although some languages have equality patterns, there are non-trivial practical considerations that make them troublesome. Which equality? Physical equality (== in Ocaml), structural equality (= in Ocaml), or some type-dependent custom equality? Furthermore, in Ocaml, there is a clear syntactic indication of which names are binders and which names are reference to previously bound values: any lowercase identifier in a pattern is a binder. These two reasons explain why Ocaml does not have equality patterns baked in. The idiomatic way to express an equality pattern in Ocaml is in a guard. That way, it's immediately clear that the matching is not structural, that identifier is not bound by this pattern matching, and which equality is in use. As for ugly, that's in the eye of the beholder — as a habitual Ocaml programmer, I find equality patterns ugly (for the reasons above).

match bindings with
 | (id, value)::tail when id = identifier -> value
 | (_, _)::tail -> getValue identifier tail
 | [] -> -1

In F#, you have another possibility: active patterns, which let you pre-define guards that concern a single site in a pattern.

Answer 5

This is a common complaint, but I don't think that there's a good workaround in general; a pattern guard is usually the best compromise. In certain specific cases there are alternatives, though, such as marking literals with the [<Literal>] attribute in F# so that they can be matched against.