maybe: define "filter"

sanctuary-js · Feb 13, 2015 · 9f7efec · 9f7efec
1 parent 705fde6
commit 9f7efec
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Showing 2 changed files with 51 additions and 3 deletions.
diff --git a/index.js b/index.js
@@ -65,6 +65,11 @@
     return maybe instanceof Nothing;
   };
 
+  //  Nothing#filter :: (a -> Boolean) -> Maybe a
+  Nothing.prototype.filter = function(pred) {  // jshint ignore:line
+    return this;
+  };
+
   //  Nothing#or :: Maybe a -> Maybe a
   Nothing.prototype.or = function(maybe) {
     return maybe;
@@ -94,6 +99,11 @@
     return maybe instanceof Just && maybe.value === this.value;
   };
 
+  //  Just#filter :: (a -> Boolean) -> Maybe a
+  Just.prototype.filter = function(pred) {
+    return pred(this.value) ? this : Nothing();
+  };
+
   //  Just#or :: Maybe a -> Maybe a
   Just.prototype.or = function(maybe) {  // jshint ignore:line
     return this;

diff --git a/test/index.js b/test/index.js
@@ -9,6 +9,9 @@ var S = require('..');
 
 var eq = assert.strictEqual;
 
+var T = function() { return true; };
+var F = function() { return false; };
+
 
 describe('maybe', function() {
 
@@ -61,6 +64,24 @@ describe('maybe', function() {
       eq(nothing.equals(S.Just(42)), false);
     });
 
+    it('provides a "filter" method', function() {
+      var nothing = S.Nothing();
+      eq(nothing.filter.length, 1);
+      eq(nothing.filter(T), nothing);
+      eq(nothing.filter(F), nothing);
+
+      var m = S.Nothing();
+      var f = function(n) { return n * n; };
+      var p = function(n) { return n < 0; };
+      var q = function(n) { return n > 0; };
+
+      // FIXME: What is the name of this law?
+      assert(m.map(f).filter(p)
+             .equals(m.filter(function(x) { return p(f(x)); }).map(f)));
+      assert(m.map(f).filter(q)
+             .equals(m.filter(function(x) { return q(f(x)); }).map(f)));
+    });
+
     it('provides a "map" method', function() {
       var nothing = S.Nothing();
       eq(nothing.map.length, 1);
@@ -114,6 +135,26 @@ describe('maybe', function() {
       eq(just.equals(S.Nothing()), false);
     });
 
+    it('provides a "filter" method', function() {
+      var just = S.Just(42);
+      eq(just.filter.length, 1);
+      eq(just.filter(T), just);
+      eq(just.filter(F).equals(S.Nothing()), true);
+      eq(just.filter(function(n) { return n > 0; }), just);
+      eq(just.filter(function(n) { return n < 0; }).equals(S.Nothing()), true);
+
+      var m = S.Just(-5);
+      var f = function(n) { return n * n; };
+      var p = function(n) { return n < 0; };
+      var q = function(n) { return n > 0; };
+
+      // FIXME: What is the name of this law?
+      assert(m.map(f).filter(p)
+             .equals(m.filter(function(x) { return p(f(x)); }).map(f)));
+      assert(m.map(f).filter(q)
+             .equals(m.filter(function(x) { return q(f(x)); }).map(f)));
+    });
+
     it('provides a "map" method', function() {
       var just = S.Just(42);
       eq(just.map.length, 1);
@@ -480,9 +521,6 @@ describe('list', function() {
 
   describe('find', function() {
 
-    var T = function() { return true; };
-    var F = function() { return false; };
-
     it('returns Just the first element satisfying the predicate', function() {
       assert(S.find(T, [null]).equals(S.Just(null)));
       assert(S.find(function(n) { return n >= 0; }, [-1, 0, 1])