Merge pull request #190 from plaid/dc-maybe-to-nullable

maybe: add S.maybeToNullable

index.js

@@ -1151,6 +1151,8 @@
   //. Takes a default value and a Maybe, and returns the Maybe's value
   //. if the Maybe is a Just; the default value otherwise.
   //.
+  //. See also [`maybeToNullable`](#maybeToNullable).
+  //.
   //. ```javascript
   //. > S.fromMaybe(0, S.Just(42))
   //. 42
@@ -1164,6 +1166,26 @@
       [a, $Maybe(a), a],
       function(x, maybe) { return maybe.isJust ? maybe.value : x; });
 
+  //# maybeToNullable :: Maybe a -> Nullable a
+  //.
+  //. Returns the given Maybe's value if the Maybe is a Just; `null` otherwise.
+  //. [Nullable][] is defined in sanctuary-def.
+  //.
+  //. See also [`fromMaybe`](#fromMaybe).
+  //.
+  //. ```javascript
+  //. > S.maybeToNullable(S.Just(42))
+  //. 42
+  //.
+  //. > S.maybeToNullable(S.Nothing())
+  //. null
+  //. ```
+  S.maybeToNullable =
+  def('maybeToNullable',
+      {},
+      [$Maybe(a), $.Nullable(a)],
+      function(maybe) { return maybe.isJust ? maybe.value : null; });
+
   //# toMaybe :: a? -> Maybe a
   //.
   //. Takes a value and returns Nothing if the value is null or undefined;
@@ -3054,6 +3076,7 @@
 //. [Functor]:        https://github.com/fantasyland/fantasy-land#functor
 //. [Monad]:          https://github.com/fantasyland/fantasy-land#monad
 //. [Monoid]:         https://github.com/fantasyland/fantasy-land#monoid
+//. [Nullable]:       https://github.com/plaid/sanctuary-def#nullable
 //. [R.equals]:       http://ramdajs.com/docs/#equals
 //. [R.map]:          http://ramdajs.com/docs/#map
 //. [R.type]:         http://ramdajs.com/docs/#type

test/maybeToNullable.js

@@ -0,0 +1,39 @@
+'use strict';
+
+var throws = require('assert').throws;
+
+var eq = require('./utils').eq;
+var errorEq = require('./utils').errorEq;
+var S = require('..');
+
+
+describe('maybeToNullable', function() {
+
+  it('is a unary function', function() {
+    eq(typeof S.maybeToNullable, 'function');
+    eq(S.maybeToNullable.length, 1);
+  });
+
+  it('type checks its arguments', function() {
+    throws(function() { S.maybeToNullable(/XXX/); },
+           errorEq(TypeError,
+                   'Invalid value\n' +
+                   '\n' +
+                   'maybeToNullable :: Maybe a -> Nullable a\n' +
+                   '                   ^^^^^^^\n' +
+                   '                      1\n' +
+                   '\n' +
+                   '1)  /XXX/ :: RegExp\n' +
+                   '\n' +
+                   'The value at position 1 is not a member of ‘Maybe a’.\n'));
+  });
+
+  it('can be applied to a Nothing', function() {
+    eq(S.maybeToNullable(S.Nothing()), null);
+  });
+
+  it('can be applied to a Just', function() {
+    eq(S.maybeToNullable(S.Just(42)), 42);
+  });
+
+});