September Morn Update

Task/Generator-Exponential/JavaScript/generator-exponential-4.js

@@ -0,0 +1,155 @@
+(() => {
+    'use strict';
+
+    // main :: IO()
+    const main = () => {
+
+        // powers :: Gen [Int]
+        const powers = n =>
+            fmapGen(
+                x => Math.pow(x, n),
+                enumFrom(0)
+            );
+
+        // xs :: [Int]
+        const xs = take(10, drop(20,
+            differenceGen(
+                powers(2),
+                powers(3)
+            )
+        ));
+
+        console.log(xs);
+        // -> [529,576,625,676,784,841,900,961,1024,1089]
+    };
+
+
+    // GENERIC FUNCTIONS ----------------------------------
+
+    // Just :: a -> Maybe a
+    const Just = x => ({
+        type: 'Maybe',
+        Nothing: false,
+        Just: x
+    });
+
+    // Nothing :: Maybe a
+    const Nothing = () => ({
+        type: 'Maybe',
+        Nothing: true,
+    });
+
+    // Tuple (,) :: a -> b -> (a, b)
+    const Tuple = (a, b) => ({
+        type: 'Tuple',
+        '0': a,
+        '1': b,
+        length: 2
+    });
+
+    // differenceGen :: Gen [a] -> Gen [a] -> Gen [a]
+    function* differenceGen(ga, gb) {
+        // All values of generator stream a except any
+        // already seen in generator stream b.
+        const
+            stream = zipGen(ga, gb),
+            sb = new Set([]);
+        let xy = take(1, stream);
+        while (0 < xy.length) {
+            const [x, y] = Array.from(xy[0]);
+            sb.add(y);
+            if (!sb.has(x)) yield x;
+            xy = take(1, stream);
+        }
+    };
+
+    // drop :: Int -> [a] -> [a]
+    // drop :: Int -> Generator [a] -> Generator [a]
+    // drop :: Int -> String -> String
+    const drop = (n, xs) =>
+        Infinity > length(xs) ? (
+            xs.slice(n)
+        ) : (take(n, xs), xs);
+
+    // enumFrom :: Enum a => a -> [a]
+    function* enumFrom(x) {
+        let v = x;
+        while (true) {
+            yield v;
+            v = 1 + v;
+        }
+    }
+
+    // fmapGen <$> :: (a -> b) -> Gen [a] -> Gen [b]
+    function* fmapGen(f, gen) {
+        let v = take(1, gen);
+        while (0 < v.length) {
+            yield(f(v[0]))
+            v = take(1, gen)
+        }
+    }
+
+    // fst :: (a, b) -> a
+    const fst = tpl => tpl[0];
+
+    // Returns Infinity over objects without finite length.
+    // This enables zip and zipWith to choose the shorter
+    // argument when one is non-finite, like cycle, repeat etc
+
+    // length :: [a] -> Int
+    const length = xs =>
+        (Array.isArray(xs) || 'string' === typeof xs) ? (
+            xs.length
+        ) : Infinity;
+
+    // snd :: (a, b) -> b
+    const snd = tpl => tpl[1];
+
+    // take :: Int -> [a] -> [a]
+    // take :: Int -> String -> String
+    const take = (n, xs) =>
+        'GeneratorFunction' !== xs.constructor.constructor.name ? (
+            xs.slice(0, n)
+        ) : [].concat.apply([], Array.from({
+            length: n
+        }, () => {
+            const x = xs.next();
+            return x.done ? [] : [x.value];
+        }));
+
+    // uncons :: [a] -> Maybe (a, [a])
+    const uncons = xs => {
+        const lng = length(xs);
+        return (0 < lng) ? (
+            lng < Infinity ? (
+                Just(Tuple(xs[0], xs.slice(1))) // Finite list
+            ) : (() => {
+                const nxt = take(1, xs);
+                return 0 < nxt.length ? (
+                    Just(Tuple(nxt[0], xs))
+                ) : Nothing();
+            })() // Lazy generator
+        ) : Nothing();
+    };
+
+    // zipGen :: Gen [a] -> Gen [b] -> Gen [(a, b)]
+    const zipGen = (ga, gb) => {
+        function* go(ma, mb) {
+            let
+                a = ma,
+                b = mb;
+            while (!a.Nothing && !b.Nothing) {
+                let
+                    ta = a.Just,
+                    tb = b.Just
+                yield(Tuple(fst(ta), fst(tb)));
+                a = uncons(snd(ta));
+                b = uncons(snd(tb));
+            }
+        }
+        return go(uncons(ga), uncons(gb));
+    };
+
+    // MAIN ---
+    return main();
+})();