isImmutable created - but need a better name

src/immutable.js

@@ -1,6 +1,22 @@
 'use strict'
 
+// The interface all immutable objects implement
+var interfaceFns = ['get', 'has', 'assoc', 'dissoc']
+
+// Predicate that indicates whether an object is
+var isImmutable = function(o){
+
+    if ( typeof o !== 'object' || o === null ) return false
+
+    for ( var i = 0, len = interfaceFns.length; i < len; i += 1 )
+        if ( typeof o[interfaceFns[i]] !== 'function' ) return false
+
+    if ( o.immutable !== true ) return false
+    return true
+}
+
 module.exports = {
     object: require('./object'),
-    array: require('./array')
+    array: require('./array'),
+    isImmutable: isImmutable
 }

test/immutable-test.js

@@ -6,4 +6,32 @@ describe('immutable', function(){
 		a.equal(im.object, require('../src/object'))
 		a.equal(im.array, require('../src/array'))
 	})
+
+    describe('isImmutable', function(){
+        it('should return true on immutable objects', function(){
+            a.equal(im.isImmutable(im.object()), true)
+            a.equal(im.isImmutable(im.array()), true)
+        })
+
+        it('should return false on data that has an immutable flag', function(){
+            var fake = { immutable: true }
+            a.equal(im.isImmutable(im.object()), true)
+            a.equal(im.isImmutable(im.array()), true)
+        })
+
+        it('should require get, set, has, assoc and dissoc to be functions, along with an immutable flag', function(){
+            var noop = function(){}
+            var convincingFake = { immutable: true, get: noop, set: noop, assoc: noop, dissoc: noop, has: noop }
+
+            a.equal(im.isImmutable(convincingFake), true)
+        })
+
+        it('should return false on all js primitives', function(){
+            a.equal(im.isImmutable(null), false)
+            a.equal(im.isImmutable(undefined), false)
+            a.equal(im.isImmutable('foo'), false)
+            a.equal(im.isImmutable(1), false)
+            a.equal(im.isImmutable(true), false)
+        })
+    })
 })