OCaml function dictionary

goingstudy

这是Cornell Ocaml course的一个function dictionary，谁能给解释一下该怎么用吗？我怎么感觉那个make定义的不对是不
dictionary

1. module FunctionDict : DICTIONARY =
   
2.   struct
   
3.     type key = string
   
4.     type 'a dict = string -> 'a
   
5.     (* The function f represents the mapping in which x is mapped to
   
6.      * (f x), except for x that are not in the mapping, in which case
   
7.      * f raises NotFound.
   
8.      *)
   
9.     exception NotFound
   
10.     let make () = fun _ -> raise NotFound
    
11.     let lookup (d : 'a dict) (key : string) : 'a = d key
    
12.     let insert (d : 'a dict) (k : key) (x : 'a) : 'a dict =
    
13.       fun k' -> if k = k' then x else d k'
    
14.     let map (f : 'a -> 'b) (d : 'a dict) = fun k -> f (d k)
    
15.   end

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