Maybe and Either Monads in C

monads.c

#include <stdio.h>
#include <stdbool.h>

// Maybe

typedef struct MaybeInt {
    int  just;
    bool nothing;
} MaybeInt;

const MaybeInt Nothing = (MaybeInt) { .nothing = true };

MaybeInt returnMaybe(int x) { return (MaybeInt) { .just = x, .nothing = false }; }

MaybeInt bindMaybe(MaybeInt (*fn)(int), MaybeInt m) {
    return m.nothing ? Nothing : fn(m.just);
}

void printMaybe(MaybeInt m) {
    if (m.nothing) printf("Nothing\n");
    else           printf("%d\n", m.just);
}

MaybeInt maybeFoo(int x) { return (x < 1)  ? Nothing : returnMaybe(x - 3); }
MaybeInt maybeBar(int x) { return (x > 0)  ? Nothing : returnMaybe(x + 3); }
MaybeInt maybeBaz(int x) { return (x != 2) ? Nothing : returnMaybe(x); }


// Either

typedef struct EitherInt {
    int   i;
    char* error;
} EitherInt;

EitherInt returnInt(int i)     { return (EitherInt) { .i = i, .error = NULL }; }
EitherInt returnError(char* e) { return (EitherInt) { .i = 0, .error = e };    }

EitherInt bindEither(EitherInt (*fn)(int), EitherInt m) {
    return (m.error != NULL) ? m : fn(m.i);
}

void printEither(EitherInt m) {
    if (m.error != NULL) printf("%s\n", m.error);
    else                 printf("%d\n", m.i);
}

EitherInt eitherFoo(int x) { return (x < 2)  ? returnError("too small")    : returnInt(x - 3); }
EitherInt eitherBar(int x) { return (x > 0)  ? returnError("not negative") : returnInt(x + 3); }
EitherInt eitherBaz(int x) { return (x != 2) ? returnError("must be 2")    : returnInt(x);     }

int main() {
    MaybeInt m = bindMaybe(maybeBaz, bindMaybe(maybeBar, maybeFoo(7)));
    printMaybe(m); // "Nothing"

    m = bindMaybe(maybeBaz, bindMaybe(maybeBar, maybeFoo(2)));
    printMaybe(m); // "2"


    EitherInt e = bindEither(eitherBaz, bindEither(eitherBar, eitherFoo(7)));
    printEither(e); // "not negative"

    e = bindEither(eitherBaz, bindEither(eitherBar, eitherFoo(2)));
    printEither(e); // "2"
    return 0;
}

you could also use a union:

typedef struct EitherInt {
    union {
        int   i;
        char* error;
    };
    bool is_error;
} EitherInt;