关于OCaml 99题compress函数模式匹配中smaller类型的疑问

Hey there! Let's unpack that compress function and clear up what the smaller branch is doing—this is a common pattern in OCaml list recursion, so it's great you're digging into the details.

First, let's restate the function for reference:

let rec compress (mylist : 'a list) : 'a list = 
  match mylist with 
  | a::(b::_ as t) -> if a = b then compress t else a::compress t 
  | smaller -> smaller

Breaking Down the Pattern Matches #

You already get the first branch: it targets lists with at least two elements. Here, a is the first element, b is the second, and t is the full sublist starting from b (so t is effectively the original list with the first element stripped). The logic checks for consecutive duplicates and recurses appropriately.

Now for the smaller branch: this is a catch-all pattern that matches every list that doesn't fit the first branch. That means it covers exactly two scenarios:

1. The empty list ([]): There are no elements to compress, so we just return it as-is.
2. A single-element list (e.g., [42], ["hello"]): With only one element, there's no possible consecutive duplicate, so we return the list directly.

What's the Type of smaller? #

Since smaller is matching against the input mylist (which is of type 'a list), smaller itself is also of type 'a list. It's just a variable name used to capture any list that can't be split into at least two elements—OCaml infers its type automatically based on the input.

To make it concrete:

• If mylist is [], smaller is [] (type 'a list)
• If mylist is [x], smaller is [x] (type 'a list)

This catch-all branch is a clean way to handle the base cases of the recursion—when the list is too short to have duplicates, we stop recursing and return the list directly.

内容的提问来源于stack exchange，提问作者Adam Ralphus