build_your_own_lisp/chapter_08/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


/* Declare new lval struct */
typedef struct {
    int type;
    long num;
    int err;
} lval;


/* Create enumeration of possible lval types */
enum {LVAL_NUM, LVAL_ERR};


/* Create enumeration of possible error types */
enum {LERR_DIV_ZERO, LERR_BAD_OP, LERR_BAD_NUM};


/* Create a new number of type lval */
lval lval_num(long x) {
    lval v;
    v.type = LVAL_NUM;
    v.num = x;
    return v;
}


/* Create a new error type lval */
lval lval_err(int x) {
    lval v;
    v.type = LVAL_ERR;
    v.err = x;
    return v;
}


/* Print an lval */
void lval_print(lval v) {
    switch (v.type) {
        /* In the case the type is a number print it
           Then break out of the switch */
        case LVAL_NUM:
            printf("%li", v.num);
            break;
        /* In the case the type is an error */
        case LVAL_ERR:
            /* Check what type of error it is & print it */
            if (v.err == LERR_DIV_ZERO) {
                printf("Error: Division by zero!");
            }
            if (v.err == LERR_BAD_OP) {
                printf("Error: Invalid operator!");
            }
            if (v.err == LERR_BAD_NUM) {
                printf("Error: Invalid number!");
            }
            break;
    }
}


/* Print an lval followed by a newline */
void lval_println(lval v) {
    lval_print(v);
    putchar('\n');
}


long min(long x, long y) {
    if (x <= y) {
        return x;
    } else {
        return y;
    }
}


long max(long x, long y) {
    if (x >= y) {
        return x;
    } else {
        return y;
    }
}


/* Use operator string to see which operation to perform */
lval eval_op(lval x, char* op, lval y) {

    /* If either value is an error return it */
    if (x.type == LVAL_ERR) {
        return x;
    }
    if (y.type == LVAL_ERR) {
        return y;
    }

    /* Otherwise do maths on the number values */

    if (strcmp(op, "+") == 0) {return lval_num(x.num + y.num);}
    if (strcmp(op, "-") == 0) {return lval_num(x.num - y.num);}
    if (strcmp(op, "*") == 0) {return lval_num(x.num * y.num);}
    if (strcmp(op, "/") == 0) {
        /* If second operand is zero return error */
        return y.num == 0 ? lval_err(LERR_DIV_ZERO)
                          : lval_num(x.num / y.num);
    }
    if (strcmp(op, "%") == 0) {return lval_num(x.num % y.num);}
    if (strcmp(op, "^") == 0) {return lval_num(pow(x.num, y.num));}
    if (strcmp(op, "min") == 0) {return lval_num(min(x.num, y.num));}
    if (strcmp(op, "max") == 0) {return lval_num(max(x.num, y.num));}

    return lval_err(LERR_BAD_OP);
}


lval eval(mpc_ast_t* t) {

    if (strstr(t->tag, "number")) {
        /* Check if there is some error in conversion */
        errno = 0;
        long x = strtol(t->contents, NULL, 10);
        return errno != ERANGE ? lval_num(x) : lval_err(LERR_BAD_NUM);
    }

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

    /* We store the third children in x */
    lval 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 : '+' | '-' | '*' | '/' | '%' | '^' |      \
            \"min\" | \"max\" ;                                 \
            expr     : <number> | '(' <operator> <expr>+ ')' ;  \
            lispy    : /^/ <operator> <expr>+ /$/ ;             \
        ",
        Number, Operator, Expr, Lispy);

    /* Print version and exit information */
    puts("Lispy version 0.0.0.0.4");
    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 */
            lval result = eval(r.output);
            lval_println(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;
}