Tree operations in the Prolog

Accustomed to c,java this common programming language, the first to understand the Prolog really some adaptation, it only needs you to tell the computer what to do, without telling the computer how to do.

The problems encountered are as follows:

For this question we consider binary expression-trees whose leaves are the "form tree" (either, Num, empty) where empty M is a number, or tree (empty, z, empty) in which case we'll have the letter Z as a kind of "variable". Every either a leaf or of the form tree (L, Op, R) where L and R are the left and right subtrees, and OP is one The arithmetic operators ' + ', '-', ' * ', '/' (signifying addition, subtraction, multiplication and division).

Write a predicate tree_eval (Value, tree, eval) which binds eval to the "result of evaluating" expression-tree tree, with The variable z set equal to the specified Value. For example:

?-Tree_eval (2, Empty,z,empty,
                 ' + ', tree (empty,1,empty),
                      '/', Tree (empty,z,empty)), Eval.
Eval = 2.5;
False.

? -Tree_eval (5, tree (empty,z,empty),
                 ' + ', tree (empty,1,empty),
                      '/', Tree (empty,z,empty)), Eval.
Eval = 5.2;
False.

There are three kinds of situations in this question.

Let's list some specific situations
　　
Tree_eval (num, tree (empty,x,empty), eval):-X=z, Eval is Num.
Tree_eval (Num, Tree (empty,x,empty), eval):-Number (x), eval is x.

These can be seen as the end conditions of recursion.

In the third case:

%case 3, left or right are a tree calcuate the node ' s children.
Tree_eval (Num, Tree (left,x,right), eval):-
Tree_eval (Num, left, Lefteval),
Tree_eval (Num, right, Righteval),
X = '-',
The Eval is lefteval-righteval.

Other operators, etc.