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)
[JS Compose] 5. Create types with Semigroups
原文：http://www.cnblogs.com/Answer1215/p/6184474.html