OZ/Mozart算术表达式求值：实现Eval函数需求问询

No problem at all—this is a classic use case for OZ's strong pattern matching capabilities, which makes handling tree-structured expressions straightforward. Let's break down how to build this Eval function step by step.

Core Approach #

We'll use a recursive function that matches against the three possible expression types:

• Base case: When we hit an int(N) tuple, we just return the integer N directly.
• Recursive cases: For add(X Y) and mul(X Y), we recursively evaluate the sub-expressions X and Y, then apply the corresponding arithmetic operation to their results.

The Code #

Here's the complete implementation:

fun {Eval Exp}
   case Exp of
      int(N) then N
    [] add(X Y) then {Eval X} + {Eval Y}
    [] mul(X Y) then {Eval X} * {Eval Y}
   end
end

How It Works #

Let's walk through each part:

• {Eval Exp} defines the function that takes your expression tuple as input.
• The case statement checks the structure of Exp:
  • When matching int(N), we immediately return N—this stops the recursion and gives us a concrete value.
  • For add(X Y), we first evaluate X and Y by calling {Eval X} and {Eval Y}, then add the two results together.
  • For mul(X Y), we do the same recursive evaluation of X and Y, then multiply their results.

Testing the Example #

Let's run your sample expression to verify:

{Browse {Eval add(int(1) mul(int(3) int(4)))} }

This will output 13, just as expected. The function first evaluates mul(int(3) int(4)) to get 12, then adds 1 to that result.

内容的提问来源于stack exchange，提问作者Alma A