add getIntercalateSemigroup

gcanti · Feb 13, 2020 · 5432531 · 5432531
1 parent 77157b1
commit 5432531
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Showing 2 changed files with 27 additions and 1 deletion.
diff --git a/src/Semigroup.ts b/src/Semigroup.ts
@@ -177,3 +177,22 @@ export const semigroupString: Semigroup<string> = {
 export const semigroupVoid: Semigroup<void> = {
   concat: () => undefined
 }
+
+/**
+ * You can glue items between and stay associative
+ *
+ * @example
+ * import { getIntercalateSemigroup, semigroupString } from 'fp-ts/lib/Semigroup'
+ *
+ * const S = getIntercalateSemigroup(' ')(semigroupString)
+ *
+ * assert.strictEqual(S.concat('a', 'b'), 'a b')
+ * assert.strictEqual(S.concat(S.concat('a', 'b'), 'c'), S.concat('a', S.concat('b', 'c')))
+ *
+ * @since 2.5.0
+ */
+export function getIntercalateSemigroup<A>(a: A): (S: Semigroup<A>) => Semigroup<A> {
+  return S => ({
+    concat: (x, y) => S.concat(x, S.concat(a, y))
+  })
+}
diff --git a/test/Semigroup.ts b/test/Semigroup.ts
@@ -13,7 +13,8 @@ import {
   semigroupString,
   semigroupSum,
   semigroupVoid,
-  getDualSemigroup
+  getDualSemigroup,
+  getIntercalateSemigroup
 } from '../src/Semigroup'
 
 describe('Semigroup', () => {
@@ -71,4 +72,10 @@ describe('Semigroup', () => {
     const S = getDualSemigroup(semigroupString)
     assert.deepStrictEqual(S.concat('a', 'b'), 'ba')
   })
+
+  it('getIntercalateSemigroup', () => {
+    const S = getIntercalateSemigroup(' ')(semigroupString)
+    assert.strictEqual(S.concat('a', 'b'), 'a b')
+    assert.strictEqual(S.concat(S.concat('a', 'b'), 'c'), S.concat('a', S.concat('b', 'c')))
+  })
 })