C monad example

monad.c

#define _GNU_SOURCE

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

typedef void *(*aquire) (void *context);

struct IO {
	aquire aquire;
	void *context;
};

typedef struct IO (*transform) (void *context);

void *io_ident (void *context) {
	return context;
}

struct IO io_return (void *context) {
	struct IO res = {
		.aquire = io_ident,
		.context = context,
	};
	return res;
}

struct io_bind_context {
	struct IO in;
	transform transform;
};

void *io_bind_aquire (void *context_) {
	struct io_bind_context *context = context_;
	if (!context->in.aquire) {
		return NULL;
	}

	struct IO res;
	res = context->transform (context->in.aquire (context->in.context));
	if (!res.aquire) {
		printf ("Return\n");
		return NULL;
	}

	return res.aquire (res.context);
}

struct IO io_bind (struct IO in, transform transform) {
	struct io_bind_context *context = malloc (sizeof (*context));
	context->in = in;
	context->transform = transform;
	
	struct IO res = {
		.aquire = io_bind_aquire,
		.context = context
	};
	return res;
}

void *do_print (void *context) {
	printf ("%s\n", (char *) context);
	return NULL;
}

struct IO io_print (char *value) {
	struct IO res = {
		.aquire = do_print,
		.context = value
	};
	return res;
}

void *do_read_line (void *context) {
	(void) context;
	char *in = malloc (1024);
	if (!fgets (in, 1024, stdin)) {
		free (in);
		return NULL;
	}
	in[strlen (in) - 1] = 0;
	return in;
}

struct IO io_read_line () {
	struct IO res = {
		.aquire = do_read_line,
		.context = NULL
	};
	return res;
}

struct IO f (void *context) {
	char *buf;
	int res;
	res = asprintf(&buf, "Sein Name ist '%s'", (char *) context);
	(void) res;
	return io_print (buf);
}

struct IO r (void *context) {
	(void) context;
	return io_read_line ();
}

int main() {
	struct IO p = io_print ((char *) "Wie ist dein Name?");
	struct IO res = io_bind (io_bind (p, r), f);
	res.aquire (res.context);
}