Understanding bind function

Question

In this article, the author explains monad using this example (I am guessing Haskell is used):

bind f' :: (Float,String) -> (Float,String)

which implies that

bind :: (Float -> (Float,String)) -> ((Float,String) -> (Float,String))

and proceed to ask to implement the function bind and offer the solution as:

bind f' (gx,gs) = let (fx,fs) = f' gx in (fx,gs++fs)

I am having problem understanding the solution. What would this look like in C or Swift?

I have gone as far as I can implementing the example and I am stuck at implementing bind:

let f: Float -> Float = { value in return 2 * value }
let g: Float -> Float = { value in return 10 + value }

let ff: Float -> (Float, String) = { value in return (f(value), "f called") }
let gg: Float -> (Float, String) = { value in return (g(value), "f called") }

Answer 1

In C++ I think it would look something like this:

#include <functional>
#include <string>
#include <utility>

using P = std::pair<float, std::string>;
using FP = std::function<P(P)>;

FP mbind(std::function<P(float)> f) {
    return [f](P in) {
        auto && res = f(in.first);
        return {res.first, in.second + res.second};
    };
}

In C you could do something similar by storing function pointers, though the invocation syntax would have to be more verbose since you'll need to pass the state around explicitly.

Answer 2

In Swift, perhaps something like this:

let bind: (Float -> (Float, String)) -> ((Float, String) -> (Float, String)) = { 
    lhs in

    return { 
        rhs in

        let result = lhs(rhs.0)
        return (result.0, "\(result.1); \(rhs.1)" )
    }
}