#include <stdlib.h>                            //  Expr.c
#include <stdio.h>
#include <string.h>
typedef struct ExprStruct * Expr;

typedef struct {   Expr left;
                            char op[4];          // only short operators!
                            Expr right;                                                  } BinRec;

typedef enum { tagNum, tagVar, tagBin } Tag;

struct ExprStruct {  //@{}@XPCL{record containing @B tagged union}
  Tag tag;
  union {           long int num;  // for tagNum
                        char * name;   // for tagVar
                        BinRec bin;    // for tagBin
            } u;                                                   //@{}@XPCL{@B {Note the @CP {struct} field label}  ``@CP {u}''}
};
Expr exprInt(long int n) {
  Expr result = malloc(sizeof(struct ExprStruct));
  if ( result == NULL) return NULL;
  result->tag = tagNum;
  result->u.num = n;
  return result;
}
Expr exprVar(char * ident) {
  Expr result = malloc(sizeof(struct ExprStruct));
  if ( result == NULL) return NULL;
  result->tag = tagVar;
  result->u.name = strdup(ident);
  return result;
}

Expr exprBin(char * op, Expr e1, Expr e2) {
  if ( op == NULL || strlen(op) > 3 ) return NULL;
  Expr result = malloc(sizeof(struct ExprStruct));
  if ( result == NULL ) return NULL;
  result->tag = tagBin;
  result->u.bin.left = e1;
  result->u.bin.right = e2;
  strcpy(result->u.bin.op, op);
  return result;
}
long int exprEval(Expr e) {
  switch (e->tag) {
    case tagNum:     return e->u.num;
    case tagBin: {
               long int val1 = exprEval(e->u.bin.left);
               long int val2 = exprEval(e->u.bin.right);
               switch ( e->u.bin.op[0] ) {                // only for demonstration!
                 case '+':    return val1 + val2;
                 case '-':     return val1 - val2;
                 case '*':    return val1 * val2;
                 case '/':     return val1 / val2;
                 default:   fprintf(stderr,"exprEval: illegal operator `%s'\n", e->u.bin.op);
              }}
              break;
    case tagVar: fprintf(stderr,"exprEval: unexpected variable `%s'\n", e->u.name);
                         break;
    default:          fprintf(stderr,"exprEval: illegal tag\n");
  }    exit(1);    }                                            // all error exit goes through this