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semigroup-concatAll-initial-value.md

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Question

By definition concat combines merely two elements of A every time. Is it possible to combine any number of them?

The concatAll function takes:

  • an instance of a semigroup
  • an initial value
  • an array of elements
import * as S from 'fp-ts/Semigroup'
import * as N from 'fp-ts/number'

const sum = S.concatAll(N.SemigroupSum)(2)

console.log(sum([1, 2, 3, 4])) // => 12

Why do I need to provide an initial value?

Answer

The concatAll method has to return an element of type A. If the provided array of elements is empty, we don't have any element of type A to return from it. Enforcing the need of an initial value makes sure we can return this initial value if the array is empty.

We could also define a concatAll method which would take a NonEmptyArray<A> and no initial value. It's actually pretty easy to implement:

import * as Semigroup from 'fp-ts/Semigroup'
import * as NEA from 'fp-ts/NonEmptyArray'

const concatAll = <A>(S: Semigroup<A>) => (as: NEA<A>) =>
  Semigroup.concatAll(S)(NEA.tail(as))(NEA.head(as))