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