parsing.c

#include <stdio.h>
#include <stdlib.h>

#include <editline/readline.h>
#include <histedit.h>

#include "mpc.h"

long eval_op(long x, char *op, long y) {
    switch (op[0]) {
    case '+':
        return x + y;
    case '-':
        return x - y;
    case '*':
        return x * y;
    case '/':
        return x / y;
    case '%':
        return x % y;
    }
    printf("Unknown operator: %s", op);
    return 0;
}

long eval(mpc_ast_t *t) {
    /* Number: arrived at leaf */
    if (strstr(t->tag, "number")) {
        return atoi(t->contents);
    }

    /* The operator always comes after '(' which is the 0'th child */
    char *op = t->children[1]->contents;

    /* We store the third child in 'x' */
    long x = eval(t->children[2]);

    /* Iterate over remaining children */
    int i = 3;
    while (strstr(t->children[i]->tag, "expr")) {
        x = eval_op(x, op, eval(t->children[i]));
        i++;
    }

    return x;
}

int main(int argc, char **argv) {
    // printf("4 * 2: %li\n", eval_op(4, "*", 2));

    /* Create some parsers */
    mpc_parser_t *Number = mpc_new("number");
    mpc_parser_t *Operator = mpc_new("operator");
    mpc_parser_t *Expr = mpc_new("expr");
    mpc_parser_t *Lispy = mpc_new("lispy");

    /* Define the parsers with the following language */
    mpca_lang(MPCA_LANG_DEFAULT,
              "                                                     \
              number   : /(-?[0-9]+)/ ;                      \
              operator : '+' | '-' | '*' | '/' | '%' ;            \
              expr     : <number> | '(' <operator> <expr>+ ')' ;  \
              lispy    : /^/ <operator> <expr>+ /$/ ;             \
            ",
              Number, Operator, Expr, Lispy);

    /* Version and exit information */
    puts("Lispy version 0.0.0.0.1\n");
    puts("Press CTRL-C to exit\n");

    puts("Example expression: * 2 2 or * (+ 1 5) (* 1 3 7)");

    while (1) {
        char *input = readline("cispy >> ");

        add_history(input);

        printf("%s\n", input);

        /* Attempt to parse the input */
        mpc_result_t r;
        if (mpc_parse("<stdin>", input, Lispy, &r)) {
            /* Evaluate */
            long result = eval(r.output);
            printf("%li\n", result);
            /* Success - print the AST */
            mpc_ast_print(r.output);
            mpc_ast_delete(r.output);
        } else {
            /* Not parsed. Print error */
            mpc_err_print(r.error);
            mpc_err_delete(r.error);
        }

        /* readline does malloc */
        free(input);
    }

    /* Undefine and delete the parsers */
    mpc_cleanup(4, Number, Operator, Expr, Lispy);

    return 0;
}