Add a basic State monad

src/teep.ts

@@ -388,6 +388,40 @@ module edc {
 
   var readerT = <A,B>(f: (A) => Monad<B>) => { return new ReaderT(f); }
 
+  class StateTuple<S,A> {
+    state: S;
+    value: A;
+    constructor(s: S, a: A) {
+      this.state = s;
+      this.value = a;
+    }
+  }
+
+  var state = <S,A>(f: (S) => StateTuple<S,A>) => { return new State(f); }
+
+  class State<S,A> {
+    f: (S) => StateTuple<S,A>;
+    constructor(f: (S) => StateTuple<S,A>) {
+      this.f = f;
+    }
+    apply(s: S): StateTuple<S,A> {
+      return this.f(s);
+    };
+    map<B>(g: (A) => B): State<S,B> {
+      return state((s: S) => {
+        var sa: StateTuple<S,A> = this.f(s);
+        return new StateTuple(sa.state, g(sa.value));
+      });
+    };
+    flatMap<B>(g: (A) => State<S,B>): State<S,B> {
+      return state((s: S) => {
+        var sa: StateTuple<S,A> = this.f(s);
+        var sb: State<S,B> = g(sa.value);
+        return sb.apply(sa.state);
+      });
+    };
+  }
+
   export var teep = {
     array:      array,
     fn:         fn,
@@ -399,6 +433,7 @@ module edc {
     read:       read,
     future:     future,
     readerT:    readerT,
+    state:      state,
   };
 
   var setExports = function () {

teep.js

@@ -357,6 +357,41 @@ var edc;
         return ReaderT;
     })();
     var readerT = function (f) { return new ReaderT(f); };
+    var StateTuple = (function () {
+        function StateTuple(s, a) {
+            this.state = s;
+            this.value = a;
+        }
+        return StateTuple;
+    })();
+    var state = function (f) { return new State(f); };
+    var State = (function () {
+        function State(f) {
+            this.f = f;
+        }
+        State.prototype.apply = function (s) {
+            return this.f(s);
+        };
+        ;
+        State.prototype.map = function (g) {
+            var _this = this;
+            return state(function (s) {
+                var sa = _this.f(s);
+                return new StateTuple(sa.state, g(sa.value));
+            });
+        };
+        ;
+        State.prototype.flatMap = function (g) {
+            var _this = this;
+            return state(function (s) {
+                var sa = _this.f(s);
+                var sb = g(sa.value);
+                return sb.apply(sa.state);
+            });
+        };
+        ;
+        return State;
+    })();
     edc.teep = {
         array: array,
         fn: fn,
@@ -367,7 +402,8 @@ var edc;
         reader: reader,
         read: read,
         future: future,
-        readerT: readerT
+        readerT: readerT,
+        state: state
     };
     var setExports = function () {
         array.map(object.keys(edc.teep), function (k) {

test/examples.js

@@ -123,22 +123,22 @@ describe('examples', function () {
         }
         return n;
       }
-  
+
       var t1 = Date.now();
       var lazyVal = teep.fn.lazy(expensiveFn)(42); // lazily apply 42 -- no computation yet
       var t2 = Date.now();
-  
+
       var t3 = Date.now();
       var slowResult = lazyVal.get(); // expensive computation the first time
       var t4 = Date.now();
       var fastResult = lazyVal.get(); // cheap cache lookup the second time
       var t5 = Date.now();
-  
+
       assert.equal(slowResult, 42);
       assert.equal(fastResult, 42);
-  
+
       assert.ok(t2 - t1 < 10); // roughly immediate
-  
+
       var slow = t4 - t3;
       var fast = t5 - t4;
       assert.ok(slow > 10 * fast); // over 10x faster
@@ -262,7 +262,7 @@ describe('examples', function () {
       ], function (x, y, z) {
         return x * (y + z);
       });
-  
+
       p.then(function (x) {
         assert.equal(x, 42);
         done();
@@ -407,15 +407,15 @@ describe('examples', function () {
         return k(x);
       };
     };
-    
+
     var asyncF = function (f) {
       return function (x) {
         return teep.future(function (k) {
           return k(f(x));
         });
       };
     };
-    
+
     var verify = function (done) {
       return function (y) {
         if (42 === y) {
@@ -495,4 +495,46 @@ describe('examples', function () {
 
   });
 
+  describe('state', function () {
+
+    var db = {
+      answer: 42,
+    };
+
+    var getAnswer = teep.state(function (db) {
+      return { state: db, value: db.answer };
+    });
+
+    var addToAnswer = function(x) {
+      return teep.state(function (db) {
+        db.answer = db.answer + x;
+        return { state: db, value: null };
+      });
+    }
+
+    it('apply', function () {
+      var result = getAnswer.apply(db);
+      assert.deepEqual(42, result.value);
+    });
+
+    it('map', function () {
+      var result =
+        getAnswer.map(function (x) {
+          return x + 1;
+        }).apply(db);
+      assert.deepEqual(43, result.value);
+    });
+
+    it('flatMap', function () {
+      var result =
+        getAnswer.flatMap(function (x) {
+          return addToAnswer(x / 2);
+        }).flatMap(function (x) {
+          return getAnswer;
+        }).apply(db);
+      assert.deepEqual((42 / 2) + 42, result.value);
+    });
+
+  });
+
 });