Data commit

Task/Monads-Writer-monad/Jsish/monads-writer-monad.jsish

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+'use strict';
+
+/* writer monad, in Jsish */
+function writerMonad() {
+
+    // START WITH THREE SIMPLE FUNCTIONS
+
+    // Square root of a number more than 0
+    function root(x) {
+        return Math.sqrt(x);
+    }
+
+    // Add 1
+    function addOne(x) {
+        return x + 1;
+    }
+
+    // Divide by 2
+    function half(x) {
+        return x / 2;
+    }
+
+
+    // DERIVE LOGGING VERSIONS OF EACH FUNCTION
+
+    function loggingVersion(f, strLog) {
+        return function (v) {
+            return {
+                value: f(v),
+                log: strLog
+            };
+        };
+    }
+
+    var log_root = loggingVersion(root, "obtained square root"),
+
+        log_addOne = loggingVersion(addOne, "added 1"),
+
+        log_half = loggingVersion(half, "divided by 2");
+
+
+    // UNIT/RETURN and BIND for the the WRITER MONAD
+
+    // The Unit / Return function for the Writer monad:
+    // 'Lifts' a raw value into the wrapped form
+    // a -> Writer a
+    function writerUnit(a) {
+        return {
+            value: a,
+            log: "Initial value: " + JSON.stringify(a)
+        };
+    }
+
+    // The Bind function for the Writer monad:
+    // applies a logging version of a function
+    // to the contents of a wrapped value
+    // and return a wrapped result (with extended log)
+
+    // Writer a -> (a -> Writer b) -> Writer b
+    function writerBind(w, f) {
+        var writerB = f(w.value),
+            v = writerB.value;
+
+        return {
+            value: v,
+            log: w.log + '\n' + writerB.log + ' -> ' + JSON.stringify(v)
+        };
+    }
+
+    // USING UNIT AND BIND TO COMPOSE LOGGING FUNCTIONS
+
+    // We can compose a chain of Writer functions (of any length) with a simple foldr/reduceRight
+    // which starts by 'lifting' the initial value into a Writer wrapping,
+    // and then nests function applications (working from right to left)
+    function logCompose(lstFunctions, value) {
+        return lstFunctions.reduceRight(
+            writerBind,
+            writerUnit(value)
+        );
+    }
+
+    var half_of_addOne_of_root = function (v) {
+        return logCompose(
+            [log_half, log_addOne, log_root], v
+        );
+    };
+
+    return half_of_addOne_of_root(5);
+}
+
+var writer = writerMonad();
+;writer.value;
+;writer.log;
+
+/*
+=!EXPECTSTART!=
+writer.value ==> 1.61803398874989
+writer.log ==> Initial value: 5
+obtained square root -> 2.23606797749979
+added 1 -> 3.23606797749979
+divided by 2 -> 1.61803398874989
+=!EXPECTEND!=
+*/