Monads/Writer monad: Difference between revisions
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# Derive Writer monad versions of each of these functions
# Apply a composition of the Writer versions of root, addOne, and half to the integer 5, deriving both a value for the Golden Ratio φ, and a concatenated log of the function applications (starting with the initial value, and followed by the application of root, etc.)
=={{header|EchoLisp}}==
Our monadic Writer elements will be pairs (string . value), where string is the log string.
<lang scheme>
(define (Writer.unit x (log #f))
(if log (cons log x)
(cons (format "init → %d" x) x)))
;; f is a lisp function
;; (Writer.lift f) returns a Writer function which returns a Writer element
(define (Writer.lift f name)
(lambda(elem)
(Writer.unit
(f (rest elem))
(format "%a \n %a → %a" (first elem) name (f (rest elem))))))
;; lifts and applies
(define (Writer.bind f elem) ((Writer.lift f (string f)) elem))
(define (Writer.print elem) (writeln 'result (rest elem)) (writeln (first elem)))
;; Writer monad versions
(define w-root (Writer.lift sqrt "root"))
(define w-half (Writer.lift (lambda(x) (// x 2)) "half"))
(define w-inc ( Writer.lift add1 "add-one"))
;; no binding required, as we use Writer lifted functions
(-> 5 Writer.unit w-root w-inc w-half Writer.print)
result 1.618033988749895
init → 5
root → 2.23606797749979
add-one → 3.23606797749979
half → 1.618033988749895
;; binding
(->> 0 Writer.unit (Writer.bind sin) (Writer.bind cos) w-inc w-half Writer.print)
result 1
init → 0
sin → 0
cos → 1
add-one → 2
half → 1
</lang>
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