Monads/Writer monad: Difference between revisions
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<lang JavaScript>(function () { |
<lang JavaScript>(function () { |
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'use strict'; |
'use strict'; |
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// START WITH THREE SIMPLE FUNCTIONS |
// START WITH THREE SIMPLE FUNCTIONS |
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// Square root of a number more than 0 |
// Square root of a number more than 0 |
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function root(x) { |
function root(x) { |
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return Math.sqrt(x); |
return Math.sqrt(x); |
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} |
} |
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// Add 1 |
// Add 1 |
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function addOne(x) { |
function addOne(x) { |
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return x + 1; |
return x + 1; |
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} |
} |
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// Divide by 2 |
// Divide by 2 |
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function half(x) { |
function half(x) { |
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return x / 2; |
return x / 2; |
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} |
} |
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// DERIVE LOGGING VERSIONS OF EACH FUNCTION |
// DERIVE LOGGING VERSIONS OF EACH FUNCTION |
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function loggingVersion(f, strLog) { |
function loggingVersion(f, strLog) { |
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return function (v) { |
return function (v) { |
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} |
} |
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} |
} |
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var log_root = loggingVersion(root, "obtained square root"), |
var log_root = loggingVersion(root, "obtained square root"), |
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log_addOne = loggingVersion(addOne, "added 1"), |
log_addOne = loggingVersion(addOne, "added 1"), |
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log_half = loggingVersion(half, "divided by 2"); |
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: |
// The Unit / Return function for the Writer monad: |
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// 'Lifts' a raw value into the wrapped form |
// 'Lifts' a raw value into the wrapped form |
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}; |
}; |
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} |
} |
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// The Bind function for the Writer monad: |
// The Bind function for the Writer monad: |
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// applies a logging version of a function |
// applies a logging version of a function |
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// to the contents of a wrapped value |
// to the contents of a wrapped value |
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// and return a wrapped result (with extended log) |
// and return a wrapped result (with extended log) |
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// Writer a -> (a -> Writer b) -> Writer b |
// Writer a -> (a -> Writer b) -> Writer b |
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function writerBind(w, f) { |
function writerBind(w, f) { |
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var writerB = f(w.value); |
var writerB = f(w.value); |
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return { |
return { |
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value: writerB.value, |
value: writerB.value, |
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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 |
// 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, |
// which starts by 'lifting' the initial value into a Writer wrapping, |
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}, writerUnit(value)); |
}, writerUnit(value)); |
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} |
} |
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var half_of_addOne_of_root = function (v) { |
var half_of_addOne_of_root = function (v) { |
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return logCompose( |
return logCompose( |
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); |
); |
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}; |
}; |
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return half_of_addOne_of_root(5); |
return half_of_addOne_of_root(5); |
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})();</lang> |
})();</lang> |
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