implements semigroup and monoid for Maybe.

origamitower · May 22, 2017 · 377d18b · 377d18b
1 parent 268af9e
commit 377d18b
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diff --git a/src/data/maybe/index.js b/src/data/maybe/index.js
@@ -25,6 +25,7 @@ module.exports = {
   Nothing: Maybe.Nothing,
   hasInstance: Maybe.hasInstance,
   of: Maybe.of,
+  empty: Maybe.empty,
   fromJSON: Maybe.fromJSON,
   [typeSymbol]: Maybe[typeSymbol],
   ['fantasy-land/of']: Maybe['fantasy-land/of'],

diff --git a/src/data/maybe/maybe.js b/src/data/maybe/maybe.js
@@ -42,6 +42,10 @@ const assertMaybe = assertType(Maybe);
 
 
 extend(Just.prototype, {
+  empty() {
+    return Nothing();
+  },
+
   /*~
    * isRequired: true
    * type: |
@@ -52,6 +56,12 @@ extend(Just.prototype, {
   }
 });
 
+extend(Nothing.prototype, {
+  empty() {
+    return Nothing();
+  },
+});
+
 
 /*~~belongsTo: Maybe */
 adtMethods(Maybe, {
@@ -173,6 +183,42 @@ adtMethods(Maybe, {
   },
 
 
+  /*~
+   * type: |
+   *   forall a: (Maybe a).(Maybe a) => Maybe a
+   */
+  concat: {
+    /*~*/
+    Nothing: function concat(aMaybe) {
+      assertMaybe('Maybe.Nothing#concat', aMaybe);
+      return aMaybe;
+    },
+
+    /*~*/
+    Just: function concat(aMaybe) {
+      assertMaybe('Maybe.Just#concat', aMaybe);
+      return aMaybe.matchWith({
+        Nothing: () => Just(this.value),
+        Just: (a) => Just(this.value.concat(a.value))
+      });
+    }
+  },
+
+
+  /*~
+   * type: |
+   *   forall a: () => Maybe a
+   */
+  empty: {
+    Nothing: function empty() {
+      return Nothing();
+    },
+    Just: function empty() {
+      return Nothing();
+    }
+  },
+
+
   /*~
    * deprecated:
    *   since: 2.0.0
@@ -232,6 +278,15 @@ Object.assign(Maybe, {
   },
 
 
+  /*~
+   * type: |
+   *   forall a: () => Maybe a
+   */
+  empty() {
+    return Nothing();
+  },
+
+
   /*~
    * deprecated:
    *   since: 2.0.0

diff --git a/test/specs-src/data.maybe.js b/test/specs-src/data.maybe.js
@@ -122,10 +122,30 @@ describe('Data.Maybe', () => {
     });
   });
 
+  describe('#concat(a)', () => {
+    property('Just(a).concat(Nothing()) = Just(a)', 'string', (a) => {
+      return Just(a).concat(Nothing()).equals(Just(a));
+    });
+
+    property('Nothing().concat(Just(a)) = Nothing()', 'string', (a) => {
+      return Nothing().concat(Just(a)).equals(Just(a));
+    });
+
+    property('Just(a).concat(Just(b)) = Just(c)', 'string', 'string', (a, b) => {
+      return Just(a).concat(Just(b)).equals(Just(a.concat(b)));
+    });
+  });
+
   describe('Fantasy Land', _ => {
     laws.Setoid(Maybe.Just);
     laws.Setoid(Maybe.Nothing);
 
+    laws.Semigroup(Maybe.Just);
+    laws.Semigroup(Maybe.Nothing);
+
+    laws.Monoid(Maybe.Just);
+    laws.Monoid(Maybe.Nothing);
+
     laws.Functor(Maybe.Just);
     laws.Functor(Maybe.Nothing);