Monads in pictures

I've seen a nice article explaining how Haskell types form a category. The objects of the category are types, an arrow a -> b is a function taking argument of type a and returning type b. A Haskell functor is then an endofunctor in this category (i.e. a functor from the category of types to itself). Unfortunately I cannot find the article anymore.

I like to think of fmap as a converting a normal function into a function that operates "through" a functor. This is clearer looking at the Haskell type signature, which can be written as:

  fmap :: (a -> b) -> (f a -> f b)

What is the relationship between fold + cons (::) and monadic bind? What is the natural transformation from fold to functorial map? Is there a sensical dual?

Thanks.

Misappropriated? Wouldn't 'grok' mean absorption of all kinds?

Personally, I like the word. It implies something more than simple comprehension or understanding. That said, I think it's misapplied here, exactly because of that difference. As an introductory tutorial, the reader is just beginning to understand the concept. It'll take much more than that to achieve what the word implies.

(I don't.)

  doThingOne(); doThingTwo()

For example, like praptak mentioned, maybe during every semicolon you can evaluate the failure state of your program and decide not to continue. I.E., "Maybe" the execution worked, and "Maybe" I have a valid computation at this point, but "Maybe" not!

The power here is that while you are really operating in this context with an implicit possibility of having failure, you can program as if each step succeeded. So now we can string together a whole sequence of possibly-failing computations, and create a bigger possibly-failing super-computation that error checks under the hood.

If you desugar everything from the State monad, than you can see, that the state 'c' is an explicit argument, which is hidden in the "sugared" version.

    import Control.Monad.State.Lazy
    
    type Counter = State Int
    
    -- using the State monad and the do notation
    increment :: Counter ()
    increment = do
       c <- get
       put $ c + 1
    
    -- using the State monad and desugar the do notation
    increment_ :: Counter ()
    increment_ = get >>= \c -> put $ c + 1
    
    -- desugar 'get'
    get_ = \c -> (c, c)
    
    -- desugar 'put'
    put_ c = \_ -> ((), c)
    
    -- desugar >>=
    bind_ a b = \c -> let (result, c') = a c in (b result) c' 
    
    -- "desugar" the State monad
    increment__ = get_ `bind_` \c -> put_ $ c + 1

Pretending that javascript is immutable, I think it would probably look something more like

    contextObj = monadBox(myObj)
    // someContextReturningFunc gets myObj as its argument from contextObj and generates newContextObj
    newContextObj = monadBind(contextObj, someContextReturningFunc)
    newNewContextObj = monadBind(newContextObj, someOtherContextReturningFunc)
    ...

    // Creating maybe "contexts"
    maybeNothing = {maybe: "nothing"}; // maybeMonad holding nothing
    maybeJust = function(x) { return {maybe: "just", just: x} }; // create a maybeMonad holding a (the '[a]' in the picture)

    // Monadic bind
    function maybeBind(maybeMonad, f) {
      // maybeMonad is '[a]' in the picture
      if (maybeMonad.maybe === "just") {
        return f(maybeMonad.just); // expects f to return another maybeMonad '[b]'
      } else {
        return maybeNothing; // also '[b]' but the b value doesn't really exist
      }
    }

    // Functor map
    function maybeMap(maybeMonad, f) {
      // maybeMonad is '[a]' in the picture
      if (maybeMonad.maybe === "just") {
        return maybeJust(f(maybeMonad.just)); // expects f to return a value without any maybe stuff ('a -> b')
        // notice that the monad wrapping remains separate from f
      } else {
        return maybeNothing;
      }
    }

    // Functor examples
    //////

    // possible values of 'f', note that maybe is not involved at all
    function addOne(x) { return x + 1; }

    // usage
    console.log(maybeMap(maybeJust(1), addOne)); // returns maybeJust(2)
    console.log(maybeMap(maybeNothing, addOne)); // returns maybeNothing


    // Monad examples
    //////
    
    // possible values for 'f', notice x is what was contained in the maybe (not the maybe itself) and the return is a maybe
    function maybeAddOneIfOdd(x) { return (x % 2 == 0) ? maybeNothing : maybeJust(x + 1); };
    function maybeAddThree(x) { return maybeJust(x + 3); };   

    // usage
    console.log(maybeBind(maybeJust(1), maybeAddThree)); // returns maybeJust(4)
    console.log(maybeBind(maybeJust(1), maybeAddOneIfOdd)); // returns maybeJust(2)
    console.log(maybeBind(maybeBind(maybeJust(1), maybeAddThree), maybeAddOneIfOdd)); // returns maybeNothing (due to 4 being even)
    console.log(maybeBind(maybeBind(maybeJust(2), maybeAddOneIfOdd), maybeAddThree)); // returns maybeNothing because the first bind returned nothing

    // and of course, composition
    function maybeMultiplyThree(x) { return maybeJust(x * 3); }
    function maybeMultiplyThreeThenAddOneIfOdd(x) {
      return maybeBind(maybeMultiplyThree(x), maybeAddOneIfOdd);
    }

    console.log(maybeBind(maybeJust(1), maybeMultiplyThreeThenAddOneIfOdd)); // returns maybeJust(4)
    console.log(maybeBind(maybeJust(2), maybeMultiplyThreeThenAddOneIfOdd)); // returns maybeNothing