Merge pull request #280 from sanctuary-js/davidchambers/void

types: define Void

index.js

@@ -531,6 +531,18 @@
   var Unknown =
   _Type (UNKNOWN, '', '', 0, always2 ('Unknown'), [], K (K (true)), []);
 
+  //# Void :: Type
+  //.
+  //. Uninhabited type.
+  //.
+  //. May be used to convey that a type parameter of an algebraic data type
+  //. will not be used. For example, a future of type `Future Void String`
+  //. will never be rejected.
+  var Void = NullaryTypeWithUrl
+    ('Void')
+    ([])
+    (K (false));
+
   //# Any :: Type
   //.
   //. Type comprising every JavaScript value.
@@ -2859,6 +2871,7 @@
     TypeClass: TypeClass,
     Undefined: Undefined,
     Unknown: Unknown,
+    Void: Void,
     env: env,
     create:
       def ('create')

test/index.js

@@ -1408,6 +1408,18 @@ Since there is no type of which all the above values are members, the type-varia
 `));
   });
 
+  test ('provides the "Void" type', () => {
+    eq ($.Void.name) ('Void');
+    eq ($.Void.url) (`https://github.com/sanctuary-js/sanctuary-def/tree/v${version}#Void`);
+    eq ($.Void.supertypes) ([]);
+
+    const isVoid = $.test ([]) ($.Void);
+    eq (isVoid (undefined)) (false);
+    eq (isVoid (null)) (false);
+    eq (isVoid (NaN)) (false);
+    eq (isVoid ('')) (false);
+  });
+
   test ('provides the "Any" type', () => {
     eq ($.Any.name) ('Any');
     eq ($.Any.url) (`https://github.com/sanctuary-js/sanctuary-def/tree/v${version}#Any`);