Monads/Writer monad: Difference between revisions
→ES5: updated writer monad to include intermediate values in log
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(→ES5: updated writer monad to include intermediate values in log) |
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<lang JavaScript>(function () {
'use strict';
// 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) {
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}
}
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
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};
}
// 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:
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,
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}, writerUnit(value));
}
var half_of_addOne_of_root = function (v) {
return logCompose(
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);
};
return half_of_addOne_of_root(5);
})();</lang>
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{{Out}}
<pre>{
"value":1.618033988749895,
"log":"Initial value: 5\n
obtained square root -> 2.23606797749979\n
added 1 -> 3.23606797749979\n
divided by 2 -> 1.618033988749895"
}</pre>
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