build_your_own_lisp/chapter_07/lispy.c

#include <stdio.h>
#include <stdlib.h>
#include "../mpc/mpc.h"


/* If we are on Windows compile these functions */
#ifdef _WIN32
#include <string.h>
#include <assert.h>

static char buffer[2048];

/* Fake readline function */
char* readline(char* prompt) {
    fputs(prompt, stdout);
    fgets(buffer, 2048, stdin);
    char* cpy = malloc(strlen(buffer)+1);
    assert(cpy != NULL)
    strcpy(cpy, buffer);
    cpy[strlen(cpy)-1] = '\0';
    return cpy;
}

/* Fake add_history function */
void add_history(char* unused) {}

/* Otherwise include the editline headers
   could use __APPLE__ for detection of OSX */
#else
#include <editline/readline.h>
#endif


/* Use operator string to see which operation to perform */
long eval_op(long x, char* op, long y) {
    if (strcmp(op, "+") == 0) {return x + y;}
    if (strcmp(op, "-") == 0) {return x - y;}
    if (strcmp(op, "*") == 0) {return x * y;}
    if (strcmp(op, "/") == 0) {return x / y;}
    return 0;
}


long eval(mpc_ast_t* t) {

    /* If tagged as number return it directly */
    if (strstr(t->tag, "number")) {
        return atoi(t->contents);
    }

    /* The operator is always second child */
    char* op = t->children[1]->contents;

    /* We store the third children in x */
    long x = eval(t->children[2]);

    /* Iterate the remaining children and combining */
    unsigned 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 const *argv[])
{
    /* 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 them with the following language */
    mpca_lang(MPCA_LANG_DEFAULT,
        "                                                       \
            number   : /-?[0-9]+/ ;                             \
            operator : '+' | '-' | '*' | '/' ;                  \
            expr     : <number> | '(' <operator> <expr>+ ')' ;  \
            lispy    : /^/ <operator> <expr>+ /$/ ;             \
        ",
        Number, Operator, Expr, Lispy);

    /* Print version and exit information */
    puts("Lispy version 0.0.0.0.3");
    puts("Press Ctrl+c to exit\n");

    /* In a never ending loop */
    while (1) {
        /* Output our prompt and get input */
        char* input = readline("lispy> ");

        /* Add input to history */
        add_history(input);

        /* Attempt to parse the user input */
        mpc_result_t r;
        if (mpc_parse("<stdin>", input, Lispy, &r)) {
            /* On success evaluate the user input */
            long result = eval(r.output);
            printf("%li\n", result);
            mpc_ast_delete(r.output);
        } else {
            /* Otherwise print the error */
            mpc_err_print(r.error);
            mpc_err_delete(r.error);
        }

        /* Free retrieved input */
        free(input);
    }

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

    return 0;
}