Add support for slice

CHANGES.md

@@ -1,4 +1,10 @@
 
+# v0.2.18
+
+-   Add support for `slice(start, end)` bindings on arrays. Strings worked
+    previously as a consequence of method dispatch. Arrays now work, but with
+    caveats.
+
 # v0.2.17
 
 -   Fix evaluator for ternary conditional operator to match the behavior of the

README.md

@@ -724,6 +724,86 @@ controller.index.add(0);
 expect(controller.view).toEqual([5, 6, 7]);
 ```
 
+Because `view(start, length)` is optimized by FRB to make minimal changes to the
+output array when `start` or `length` change,  in most cases it is better than a
+hypothetical `slice(start, start + length)` operator. There is no `slice`
+operator for arrays in FRB.
+
+### Slice
+
+**Reactive support for arrays added in version 0.2.18**
+
+The slice operator operates on both strings and arrays and produces a window
+from a `start` offset up to but not including an `end` offset.
+This operator is unusual in FRB because it operates on either arrays and
+strings.
+
+```javascript
+var object = Bindings.defineBindings({
+    start: 2,
+    end: 4
+}, {
+    slice: {"<-": "cake.slice(start, end)"}
+});
+
+object.cake = "abcdefg";
+expect(object.slice).toBe("cd");
+
+object.cake = [1, 2, 3, 4, 5, 6, 7];
+expect(object.slice).toEqual([3, 4]);
+```
+
+The behavior for strings is unimaginative.
+If the source string, or either the start index or end index changes, the target
+will be replaced with that slice of the source.
+
+The behavior for arrays is nuanced.
+To avoid these nuances, you should generally use the `view(start, length)`
+operator directly.
+It is better suited for viewing a sliding window of an array, allowing
+the position and size of the window to change orthogonally.
+
+However, if you happen to use the `slice` operator on an array, you can expect
+it to deviate from the norm.
+Most FRB operators that produce arrays will only emit that array once, even if
+the value of the input changes.
+This is beneficial because one can attach change listeners once to an output
+array knowing that these change listeners will continue to react until the
+binding is canceled, even if the input temporarily null or is replaced.
+Because the `slice` operator may have either a string or an array as its input,
+the output value will change if ever the input value is replaced and may switch
+between being an array or string depending on the input type.
+
+Altering the content of the source does not change the value of the output.
+
+```javascript
+// Continued from above...
+var original = object.slice;
+object.cake.shift();
+expect(object.slice).toEqual([4, 5]);
+expect(object.slice).toBe(original);
+```
+
+Altering the `start` or `end` parameters will also only change the content of
+the output.
+
+```javascript
+// Continued from above...
+object.start--;
+object.end++;
+expect(object.cake).toEqual([2, 3, 4, 5, 6, 7]);
+expect(object.slice).toEqual([3, 4, 5, 6]);
+expect(object.slice).toBe(original);
+```
+
+But replacing the input will produce a new output value.
+```javascript
+// Continued from above...
+object.cake = [7, 6, 5, 4, 3, 2, 1, 0];
+expect(object.slice).toEqual([6, 5, 4, 3]);
+expect(object.slice).not.toBe(original);
+```
+
 ### Enumerate
 
 An enumeration observer produces `[index, value]` pairs.  You can bind
@@ -2108,6 +2188,7 @@ Bindings.defineBindings({
 }, {
     b: {
         "<-": "a + 1",
+        trace: true
     }
 });
 ```

bind.js

@@ -203,7 +203,7 @@ function bindRangeContent(
                 source.swap(0, source.length, target);
                 isActive = false;
                 cancel = establishRangeContentBinding();
-            } else if (!source && !isActive) {
+            } else if (!source && !isActive && target.clone) {
                 trace && console.log("RANGE CONTENT TARGET INITIALIZED TO COPY OF SOURCE", trace.targetPath, "<-", tarce.sourcePath, "WITH", source);
                 assignSource(target.clone(), sourceScope);
             }
@@ -223,7 +223,7 @@ function bindRangeContent(
                 target.swap(0, target.length, source);
                 isActive = false;
                 cancel = establishRangeContentBinding();
-            } else if (!target) {
+            } else if (!target && source.clone) {
                 assignTarget(source.clone(), targetScope);
             }
         }

compile-observer.js

@@ -45,6 +45,7 @@ var semantics = compile.semantics = {
         flatten: Observers.makeFlattenObserver,
         concat: Observers.makeConcatObserver,
         view: Observers.makeViewObserver,
+        slice: Observers.makeSliceObserver,
         sum: Observers.makeSumObserver,
         average: Observers.makeAverageObserver,
         last: Observers.makeLastObserver,

observers.js

@@ -971,6 +971,27 @@ function makeReplacingViewObserver(observeInput, observeStart, observeLength) {
     };
 }
 
+exports.makeSliceObserver = makeSliceObserver;
+function makeSliceObserver(observeInput, observeStart, observeEnd) {
+    var observeLength = makeSubObserver(observeEnd, observeStart);
+    var observeArrayView = makeReplacingViewObserver(observeValue, makeParentObserver(observeStart), makeParentObserver(observeLength));
+    var observeStringSlice = makeStringSliceObserver(observeValue, makeParentObserver(observeStart), makeParentObserver(observeEnd));
+    return function (emit, scope) {
+        return observeInput(function (input) {
+            if (typeof input === "string") {
+                return observeStringSlice(emit, scope.nest(input));
+            } else if (Array.isArray(input)) {
+                return observeArrayView(emit, scope.nest(input));
+            } else {
+                return emit();
+            }
+        }, scope)
+    };
+}
+
+var makeStringSliceObserver = makeMethodObserverMaker("slice");
+var makeSubObserver = makeOperatorObserverMaker(Operators.sub);
+
 var observeZero = makeLiteralObserver(0);
 
 exports.makeEnumerateObserver = makeNonReplacing(makeReplacingEnumerateObserver);

spec/evaluate.js

@@ -304,6 +304,26 @@ module.exports = [
         output: [3, 4]
     },
 
+    {
+        path: "source.slice(start, end)",
+        input: {
+            source: [1, 2, 3, 4, 5],
+            start: 1,
+            end: 3
+        },
+        output: [2, 3]
+    },
+
+    {
+        path: "source.slice(start, end)",
+        input: {
+            source: "abcdefg",
+            start: 1,
+            end: 3
+        },
+        output: "bc"
+    },
+
     {
         path: "a && b",
         input: {a: true, b: true},

spec/readme-spec.js

@@ -470,6 +470,40 @@ describe("Tutorial", function () {
         expect(controller.view).toEqual([5, 6, 7]);
     });
 
+    it("Slice", function () {
+        var object = Bindings.defineBindings({
+            start: 2,
+            end: 4
+        }, {
+            slice: {"<-": "cake.slice(start, end)"}
+        });
+
+        object.cake = "abcdefg";
+        expect(object.slice).toBe("cd");
+
+        object.cake = [1, 2, 3, 4, 5, 6, 7];
+        expect(object.slice).toEqual([3, 4]);
+
+        // Continued from above...
+        var original = object.slice;
+        object.cake.shift();
+        expect(object.slice).toEqual([4, 5]);
+        expect(object.slice).toBe(original);
+
+        // Continued from above...
+        object.start--;
+        object.end++;
+        expect(object.cake).toEqual([2, 3, 4, 5, 6, 7]);
+        expect(object.slice).toEqual([3, 4, 5, 6]);
+        expect(object.slice).toBe(original);
+
+        // Continued from above...
+        object.cake = [7, 6, 5, 4, 3, 2, 1, 0];
+        expect(object.slice).toEqual([6, 5, 4, 3]);
+        expect(object.slice).not.toBe(original);
+
+    });
+
     it("Enumerate", function () {
         var object = {letters: ['a', 'b', 'c', 'd']};
         bind(object, "lettersAtEvenIndexes", {