Build a Cross‑Platform C REPL with GNU Readline and MPC Parser

Interactive REPL

Parsing‑oriented programming languages require a parser to translate source code into executable instructions. Implementing such a language in C involves four main parts: designing syntax and semantics, implementing the parser, implementing the executor, and providing utility libraries.

Design syntax and semantics : This tutorial focuses on a Lisp‑like syntax called Doge .

Implement the parser : We first use a simple loop with fgets and printf, then replace them with GNU Readline for line editing and history.

Implement the executor : The C program itself executes the input.

Provide utility libraries : Additional helper functions can be added as needed.

Basic REPL Implementation

# mkdir lispy
$ cd lispy
$ code . // use VS Code IDE

#include <stdio.h>
#define INPUT_BUFF_SIZE 2048 // input buffer size
static char input[INPUT_BUFF_SIZE];
int main(int argc, char *argv[]) {
    puts("Lispy Version 0.1");
    puts("Press Ctrl+c to Exit
");
    while (1) {
        fputs("lispy> ", stdout);
        fgets(input, INPUT_BUFF_SIZE, stdin);
        printf("You're a %s", input);
    }
    return 0;
}

Running the program yields basic input/output functionality, but arrow keys produce escape sequences like ^[[D on Unix systems.

Using GNU Readline

GNU Readline adds line editing and history support.

#include <stdio.h>
#include <stdlib.h>
#include <readline/readline.h>
#include <readline/history.h>
int main(int argc, char *argv[]) {
    puts("Lispy Version 0.1");
    puts("Press Ctrl+c to Exit
");
    while (1) {
        char *input = readline("lispy> ");
        add_history(input);
        printf("You're a %s
", input);
        free(input);
    }
    return 0;
}

Compile with:

gcc -std=c99 -Wall main.c -o main -lreadline

Portability with Preprocessor Directives

To make the code compile on Windows, Linux, and macOS, we use conditional compilation to provide fake readline and add_history functions on Windows and include the real library on Unix‑like systems.

#include <stdio.h>
#include <stdlib.h>
#ifdef _WIN32
#include <string.h>
static char buffer[2048];
char *readline(char *prompt) {
    fputs(prompt, stdout);
    fgets(buffer, 2048, stdin);
    char *cpy = malloc(strlen(buffer) + 1);
    strcpy(cpy, buffer);
    cpy[strlen(cpy) - 1] = '\0';
    return cpy;
}
void add_history(char *unused) {}
#else
#ifdef __linux__
#include <readline/readline.h>
#include <readline/history.h>
#endif
#ifdef __MACH__
#include <readline/readline.h>
#endif
#endif
int main(int argc, char *argv[]) {
    puts("Lispy Version 0.1");
    puts("Press Ctrl+c to Exit
");
    while (1) {
        char *input = NULL;
        input = readline("lispy> ");
        add_history(input);
        printf("You're a %s
", input);
        free(input);
    }
    return 0;
}

Using the MPC Library to Build a Parser

MPC (Micro Parser Combinators) simplifies creating parsers for custom languages.

/* Build a parser 'Adjective' */
mpc_parser_t *Adjective = mpc_or(4,
    mpc_sym("wow"), mpc_sym("many"),
    mpc_sym("so"),  mpc_sym("such")
);
/* Build a parser 'Noun' */
mpc_parser_t *Noun = mpc_or(5,
    mpc_sym("lisp"), mpc_sym("language"),
    mpc_sym("book"), mpc_sym("build"),
    mpc_sym("c")
);
/* Phrase = Adjective followed by Noun */
mpc_parser_t *Phrase = mpc_and(2, mpcf_strfold, Adjective, Noun, free);
/* Doge = zero or more Phrases */
mpc_parser_t *Doge = mpc_many(mpcf_strfold, Phrase);

Example Doge sentences:

"wow book such language many lisp"
"so c such build such language"
"many build wow c"
""
"wow lisp wow c many language"
"so c"

A more concise definition uses mpca_lang with a grammar string:

mpc_parser_t *Adjective = mpc_new("adjective");
mpc_parser_t *Noun      = mpc_new("noun");
mpc_parser_t *Phrase    = mpc_new("phrase");
mpc_parser_t *Doge      = mpc_new("doge");
mpca_lang(MPCA_LANG_DEFAULT,
  "adjective : \"wow\" | \"many\" | \"so\" | \"such\";
"
  "noun      : \"lisp\" | \"language\" | \"book\" | \"build\" | \"c\";
"
  "phrase    : <adjective> <noun>;
"
  "doge      : <phrase>*;",
  Adjective, Noun, Phrase, Doge);
mpc_cleanup(4, Adjective, Noun, Phrase, Doge);