September 2017 Update

Task/Binary-digits/JavaScript/binary-digits-3.js

@@ -0,0 +1,47 @@
+(() => {
+
+    // showBin :: Int -> String
+    const showBin = n => {
+        const binaryChar = n => n !== 0 ? '一' : '〇';
+
+        return showIntAtBase(2, binaryChar, n, '');
+    };
+
+    // showIntAtBase :: Int -> (Int -> Char) -> Int -> String -> String
+    const showIntAtBase = (base, toChr, n, rs) => {
+        const showIt = ([n, d], r) => {
+            const r_ = toChr(d) + r;
+            return n !== 0 ? (
+                showIt(quotRem(n, base), r_)
+            ) : r_;
+        };
+        return base <= 1 ? (
+            'error: showIntAtBase applied to unsupported base'
+        ) : n < 0 ? (
+            'error: showIntAtBase applied to negative number'
+        ) : showIt(quotRem(n, base), rs);
+    };
+
+    // quotRem :: Integral a => a -> a -> (a, a)
+    const quotRem = (m, n) => [Math.floor(m / n), m % n];
+
+    // GENERIC FUNCTIONS FOR TEST ---------------------------------------------
+
+    // intercalate :: String -> [a] -> String
+    const intercalate = (s, xs) => xs.join(s);
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // unlines :: [String] -> String
+    const unlines = xs => xs.join('\n');
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x);
+
+    // TEST -------------------------------------------------------------------
+
+    return unlines(map(
+        n => intercalate(' -> ', [show(n), showBin(n)]), [5, 50, 9000]
+    ));
+})();