为什么OCaml的模式匹配比Erlang的弱？

5lwkijsr 于 2022-12-08 发布在 Erlang

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I am new to OCaml and reading the Real World OCaml (RWO) book. Pattern matching is described in chapter 3 and seems weak in comparison to Erlang's (or Prolog's) pattern matching.
My questions are:

why is OCaml's pattern matching weaker?
are there any advantages to OCaml's style of pattern matching?
A concrete example:
The following function (taken from RWO p. 63) destutters a list

let rec destutter list =
    match list with
    | [] -> []
    | [hd] -> [hd]
    | hd :: hd' :: tl ->
      if hd = hd' then ds1 (hd' :: tl)
      else hd :: ds1 (hd' :: tl)
  ;;
# destutter [1;2;3;3;4;5;5;6];;
- : int list = [1; 2; 3; 4; 5; 6]

In Erlang it would be possible (and I think preferred) to use pattern matching instead of the conditional:

destutter([])      -> [];
destutter([X])     -> [X];
destutter([H,H|T]) -> destutter([H|T]);
destutter([H|T])   -> [H | destutter(T)].

Trying this kind of thing in OCaml ...

let rec destutter list =
    match list with
    | [] -> []
    | [hd] -> [hd]
    | hd :: hd :: tl -> destutter tl  (* error *)
    | hd :: tl -> hd :: destutter tl
  ;;

... raises an error on the marked line:

Error: Variable hd is bound several times in this matching

So, Erlang/Prolog-style pattern matching doesn't work in OCaml. Why? What are the advantages of the OCaml approach?
With thanks and best wishes
Ivan

erlang

来源：https://stackoverflow.com/questions/34956003/why-is-ocamls-pattern-matching-weaker-than-erlangs

3条答案

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j1dl9f461#

First, you can always capture equality between pattern variables via a when clause in OCaml, e.g.:

let rec destutter = function
    | []              -> []
    | [hd]            -> [hd]
    | hd :: hd' :: tl
      when hd = hd'   -> destutter (hd :: tl)
    | hd :: hd' :: tl -> hd :: destutter (hd' :: tl)

There is a trade-off here. While Erlang is more expressive, OCaml pattern matching is simpler (which means a simpler language definition, compiler, etc.), and you still can do what you need (at the cost of writing more code).
Note that while you can rewrite a non-linear pattern as a linear pattern with a when clause, that is not the primary issue. More critically, pattern matching then needs to have a notion of equality for arbitrary types in order to support non-linear patterns. This is not an issue in Erlang, but OCaml not only already has = vs == built-in (structural equality vs. identity), but for any given type, it may not be the kind of equality that you need (think strings and case-sensitivity, for example). Then, as a consequence, checking for exhaustiveness or overlap becomes non-trivial. In the end, it's questionable whether providing a special case for one specific type of equality is worth it, given how many useful relations between parts of a pattern there can be. (I note that non-strict languages have additional issues.)
As an aside, Prolog's pattern matching is unification-based and strictly more powerful than either Erlang or OCaml (but also more difficult to implement).

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zy1mlcev2#

Patterns in OCaml are compiled into a very efficient code with lots of sophisticated optimizations . Bjarne Stroustrup even bragged that they managed to write something comparable in certain cases in C++. But in general OCaml pattern matching is much faster. And it's charming to look at the assembly output. And maybe Erlang provides more flexibility, but it is expected from a dynamic language. Otherwise, why use them at all.
There is another issue also. Patterns are matched structurally. If you want to match [H,H|T] you are actually invoking the comparison of the first two elements. In most cases, the comparison function should be provided by a user, and it is not known in advance.

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rfbsl7qr3#

Erlang模式更强大，因为它可以匹配在运行时确定的内容。OCaml模式匹配在编译时确定的内容。由此可见，OCaml模式可能运行得更快。我还发现OCaml样式的模式更容易推理。

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