%{
#include <stdio.h>
%}

%token S
%token K 

%token OPBRACK
%token CLBRACK

%start comm

%%

comm: comm '\n'            { return $$;}
    | /* empty */
    | comm exp '\n'        {$$ = $2; return $$;}
    | comm error '\n'      {printf("Invalid combinator sequence\n"); return 1;}
    ;

exp: exp2                  {$$ = $1; }
   | exp exp2              {$$ = (int)mknode((Tree*)$1,(Tree*)$2); }
   ;

exp2: OPBRACK exp CLBRACK  {$$ = $2; }
    | K                    {$$ = (int)mkK(); }
    | S                    {$$ = (int)mkS(); }
    ;

%%

#include "lex.yy.c"

enum combinator { s , k };

typedef union Tree Tree;

typedef struct Node {
  Tree* left;
  Tree* right;
} Node;

union Tree {
  Node* N;
  int C;
};

Tree* mkK() {
  Tree* t = (Tree*) malloc(sizeof(Tree));
  t->C = k;
  return t;
}

Tree* mkS() {
  Tree* t = (Tree*) malloc(sizeof(Tree));
  t->C = s;
  return t;
}

Tree* mknode(Tree* t1, Tree* t2) {
  Tree* t = (Tree*) malloc(sizeof(Tree));
  Node* n = (Node*) malloc(sizeof(Node));
  n->left = t1;
  n->right = t2;
  t->N = n;
  return t;
}

yyerror(s)
  char *s;
{  printf("%s\n", s);
}