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Ensure failsafe combination using monoids

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In this video we define monoids and promote our semigroups

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In this video we define monoids and promote our semigroups

Let's consider addition. If we have 1 + 0 we get back 1. If we have 2 + 0, we get back 2. In fact if we have x + 0, we get back whatever x is.

Remember, a semigroup is a type with a concat method. If this addition is our concatenation, we have a neutral element here that acts as an identity of sorts that gives us back our element we're trying to concat with. If we have a special element like the zero here under addition, we have what's called a monoid, that is a semigroup with a special element in there that acts like a neutral identity.

Let's go ahead and define an interface with this and we'll say Sum is a monoid if it has a concat method that's semigroup, and it has this empty function on it that will return us a special type here, a special Sum with the 0 here, a neutral element.


Sum.empty = () => Sum(0)

Here, we can say Sum.empty, and we'll concat that with the rest of it. We're programming to an interface here now, with empty and concat, not special zeroes and plusses. That shall give us our results of Sum(3),

const res = Sum.empty().concat(Sum(1).concat(Sum(2)))

and it doesn't seem to matter that this empty is here at all, because it is a neutral element. We should still have Sum(3) without it. Good, good.

const res = Sum.concat(Sum(1).concat(Sum(2)))

Now let's see if we can do the same with All. All has a concat method, so it is a semigroup. If we want to give it a special element, we'll make this empty function, so it can be a monoid. What would this element be? It has to be an All of something, because we want to always hold the All structure as we're combining things.

const All = x => 
    concat: ({x: y}) =>
        All(x && y),
    inspect: () =>

We're using the Boolean with Conjunction here to combine things. Let's go ahead see what happens. If we combine true with the false, we will get back a false that's not our x. How about a true with a true? There we go. We get back true. Good.

How about if we have the other way around, we have a false and a true? We'll get back false. Perfect. So our x, combined with our neutral element, true, returns back our x here, just like 0. Now All, we know our neutral element is true here. So, there we go.

All.empty = () = All(true)

Now, if we have All concat true, we can make sure this stays All(true) there, and we can go ahead and .concat(All.empty()).

const res = All(false).concat(All(true))

This should still be All(true), and if any of these become false, it will remain false, and the empty element doesn't seem to affect anything at all. Good, good.

Finally, let's see if we can define an empty method for our first semigroup. First, we'll just throw away the second thing we're trying to combine with, and return our first. Let's see if this works. So First('hello'), and we want to concat that with some special element that returns back our first thing. This could be anything, really.

const First = x =>
    concat: _ =>
    inspect: () =>


How about the other way around? If we have some special element concated with our First('hello')


Does this work? No it doesn't because it's just going to throw our second part away and this First neutral element can't be defined. We just don't know how to do it.

For now without any special tricks, our First semigroup shall remain a semigroup. We cannot promote it to a monoid because we have no way to define a neutral element on here.

Let's look at one last thing. Suppose we want to write a sum function. It takes some list of xs and we just reduce them by adding each one together and starting at 0. Would you look at that? We have the same operations as on our Sum type.

const sum = xs =>
    xs.reduce((acc, x) => acc + x, 0)

Up here we have concat and our empty 0. If I call sum on a list of [1,2,3] it will yield 6. If I call it on an empty list, it will yield 0 for empty. Let's see if we can do the same thing with All. Sure enough we have our operation here, our conjunction and we start with true.

const all = xs =>
    xs.reduce((acc, x) => acc + x, true)

On an empty list we will receive true, otherwise it will go through and combine each one with this combining method. Finally with our first, we have a list. We will try to take the first element of the list. We just grab the accumulator in this case and we can give it no starting value because we don't have a starting value. If we call first on a list of [1,2,3] we will get back the 1.

However, if it's an empty list it will blow up, and we can witness this here. I'll console.log(first([1,2,3])). That should be 1, and an empty list it will just blow up in our faces here because we don't have any value to return.

What can we deduce from this? A semigroup, it does not have an element to return so it's not a safe operation, whereas with the monoids we could take as many as we possibly want, even none, and still return us back something. It's a perfectly safe operation here that we can reduce as many of them as we'd like.

Joel's Head
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