[JS Compose] 5. Create types with Semigroups

Posted 2020-08-23 Answer1215

An introduction to concatting items via the formal Semi-group interface. Semi-groups are simply a type with a concat method that are associative. We define three semigroup instances and see them in action.

 

A semigroup is a type with a concat method. Let‘s see if we have a string A, we can concat that with the string B. String is the semigroup here because it has a concat method. If we log this out here, we shall see the results AB and there we are.

"a".concat("b").concat("c"); //"abc"
"a".concat("b".concat("c")); //"abc"

 

We can also define our own semi-group:

const Sum = x =>
  ({
    x, // we need to export x, so we can access it
    concat: o => Sum(o.x + x), // o -> Sum(x)
    toString: () => `Sum(${x})`
  });

const res = Sum(1).concat(Sum(2));
console.log(res.toString()); // Sum(3)

const All = x => ({
  x,
  concat: o => All(o.x && x),
  toString: ()=> `All(${x})`
});

const res = All(true).concat(All(false));
console.log(res.toString()); // All(false)

const First = x => ({
  x,
  concat: o => First(x),
  toString: () => `First(${x})`
});

const res = First(true).concat(First(false));
console.log(res.toString()); // First(true)