Docs: add getDualSemigroup / getDualMonoid documentation

gcanti · May 5, 2020 · 17aa4af · 17aa4af
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diff --git a/docs/modules/Monoid.ts.md b/docs/modules/Monoid.ts.md
@@ -54,12 +54,22 @@ Added in v2.0.0
 
 # getDualMonoid
 
+The dual of a `Monoid`, obtained by swapping the arguments of `concat`.
+
 **Signature**
 
 ```ts
 export declare function getDualMonoid<A>(M: Monoid<A>): Monoid<A>
 ```
 
+**Example**
+
+```ts
+import { getDualMonoid, monoidString } from 'fp-ts/lib/Monoid'
+
+assert.deepStrictEqual(getDualMonoid(monoidString).concat('a', 'b'), 'ba')
+```
+
 Added in v2.0.0
 
 # getEndomorphismMonoid

diff --git a/docs/modules/Semigroup.ts.md b/docs/modules/Semigroup.ts.md
@@ -59,12 +59,22 @@ Added in v2.0.0
 
 # getDualSemigroup
 
+The dual of a `Semigroup`, obtained by swapping the arguments of `concat`.
+
 **Signature**
 
 ```ts
 export declare function getDualSemigroup<A>(S: Semigroup<A>): Semigroup<A>
 ```
 
+**Example**
+
+```ts
+import { getDualSemigroup, semigroupString } from 'fp-ts/lib/Semigroup'
+
+assert.deepStrictEqual(getDualSemigroup(semigroupString).concat('a', 'b'), 'ba')
+```
+
 Added in v2.0.0
 
 # getFirstSemigroup

diff --git a/src/Monoid.ts b/src/Monoid.ts
@@ -112,6 +112,13 @@ export function getTupleMonoid<T extends ReadonlyArray<Monoid<any>>>(
 }
 
 /**
+ * The dual of a `Monoid`, obtained by swapping the arguments of `concat`.
+ *
+ * @example
+ * import { getDualMonoid, monoidString } from 'fp-ts/lib/Monoid'
+ *
+ * assert.deepStrictEqual(getDualMonoid(monoidString).concat('a', 'b'), 'ba')
+ *
  * @since 2.0.0
  */
 export function getDualMonoid<A>(M: Monoid<A>): Monoid<A> {

diff --git a/src/Semigroup.ts b/src/Semigroup.ts
@@ -59,6 +59,13 @@ export function getTupleSemigroup<T extends ReadonlyArray<Semigroup<any>>>(
 }
 
 /**
+ * The dual of a `Semigroup`, obtained by swapping the arguments of `concat`.
+ *
+ * @example
+ * import { getDualSemigroup, semigroupString } from 'fp-ts/lib/Semigroup'
+ *
+ * assert.deepStrictEqual(getDualSemigroup(semigroupString).concat('a', 'b'), 'ba')
+ *
  * @since 2.0.0
  */
 export function getDualSemigroup<A>(S: Semigroup<A>): Semigroup<A> {