#34: add update combinator; rewrite get[State]/put[State]

lib/combinators.js

@@ -126,35 +126,40 @@ function mapError(f, parser) {
     return catchError(F.compose(error, f), parser);
 }
 
-function put(xs) {
+function update(f) {
     /*
-    m t -> Parser e s (m t) ()
+    (m t -> m t) -> Parser e s (m t) (m t)
     */
-    function f(_xs_, s) {
-        return good(null, xs, s);
-    }
-    return new Parser(f);
+    checkFunction('update', f);
+    return new Parser(function(xs, s) {
+        var ys = f(xs);
+        return good(ys, ys, s);
+    });
 }
 
-function putState(s) {
+function put(xs) {
     /*
-    s -> Parser e s (m t) ()
+    m t -> Parser e s (m t) (m t)
     */
-    function f(xs, _s_) {
-        return good(null, xs, s);
-    }
-    return new Parser(f);
+    return update(F.constF(xs));
 }
 
 function updateState(g) {
     /*
     (s -> s) -> Parser e s (m t) ()
     */
     checkFunction('updateState', g);
-    function f(xs, s) {
-        return good(null, xs, g(s));
-    }
-    return new Parser(f);
+    return new Parser(function(xs, s) {
+        var newState = g(s);
+        return good(newState, xs, newState);
+    });
+}
+
+function putState(s) {
+    /*
+    s -> Parser e s (m t) s
+    */
+    return updateState(F.constF(s));
 }
 
 function check(predicate, parser) {
@@ -377,10 +382,10 @@ function sepBy0(parser, separator) {
 var zero = new Parser(function(_xs_, _s_) {return M.zero;});
 
 // Parser e s (m t) (m t)
-var get = new Parser(function(xs, s) {return good(xs, xs, s);});
+var get = update(F.id);
 
 // Parser e s (m t) s
-var getState = new Parser(function(xs, s) {return good(s, xs, s);});
+var getState = updateState(F.id);
 
 
 function Itemizer(f) {
@@ -488,9 +493,10 @@ module.exports = {
     'error'      : error,
     'catchError' : catchError,
     'mapError'   : mapError,
+    'update'     : update,
     'put'        : put,
-    'putState'   : putState,
     'updateState': updateState,
+    'putState'   : putState,
     'check'      : check,
     'many0'      : many0,
     'many1'      : many1,

lib/functions.js

@@ -44,12 +44,22 @@ function buildSet(elems) {
     return obj;
 }
 
+function constF(x) {
+    return function() { return x; }
+}
+
+function id(x) {
+    return x;
+}
+
 module.exports = {
     'compose' : compose,
     'getArgs' : getArgs,
     'first'   : first,
     'second'  : second,
     'buildSepByValue': buildSepByValue,
     'pair'    : pair,
-    'buildSet': buildSet
+    'buildSet': buildSet,
+    'constF'  : constF,
+    'id'      : id
 };

test/combinators.js

@@ -226,20 +226,26 @@ testModule('combinators', function() {
         deepEqual(v2, M.zero);
         deepEqual(v3, good(82, '123abc', null));
     });
+
+    test("Update", function() {
+        var v1 = C.update(function(x) {return x + 'qrs';}).parse('abc', 18);
+        // presents the updated "rest" as the value
+        deepEqual(v1, good('abcqrs', 'abcqrs', 18));
+    });
 
     test("Put", function() {
         var val = C.put('xyz');
-        deepEqual(val.parse('abc', []), good(null, 'xyz', []));
-    });
-
-    test("PutState", function() {
-        var v1 = C.putState(29).parse('abc123', 2);
-        deepEqual(v1, good(null, 'abc123', 29));
+        deepEqual(val.parse('abc', []), good('xyz', 'xyz', []));
     });
 
     test("UpdateState", function() {
         var v1 = C.updateState(function(x) {return x * 4;}).parse('abc', 18);
-        deepEqual(v1, good(null, 'abc', 72));
+        deepEqual(v1, good(72, 'abc', 72));
+    });
+
+    test("PutState", function() {
+        var v1 = C.putState(29).parse('abc123', 2);
+        deepEqual(v1, good(29, 'abc123', 29));
     });
 
     test("Check", function() {

test/functions.js

@@ -62,5 +62,16 @@ testModule('functions', function() {
     test("buildSet", function() {
         deepEqual(F.buildSet("abc"), {'a': 1, 'b': 1, 'c': 1});
     });
+
+    test("constF", function() {
+        deepEqual(F.constF("abc")(), "abc");
+        deepEqual(F.constF("abc")(1, 2, 3), "abc");
+        deepEqual(F.constF()(1, 2, 3), undefined);
+    });
+
+    test("id", function() {
+        deepEqual(F.id(123), 123);
+        deepEqual(F.id(), undefined);
+    });
 
 });