Monads/Writer monad: Difference between revisions

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// Monads/Writer monad . Nigel Galloway: July 20th., 2022

type Riter<'n~~,'g~~>=Riter of 'n * List<'g>

type Riter<'n>=Riter of 'n * List<string>

let bind=function |(n,g)->Riter(n,[g])

let eval=function |Riter(n,g)->(n,g)

let compose f=function |Riter(n,g)->let n,l=eval(f n) in Riter(n,List.append g l)

let initV(n~~:float)~~=~~bind~~(n,sprintf "Initial Value %f" n)

let initV n=Riter(n,[sprintf "Initial Value %f" n])

let sqrt n=~~bind~~(sqrt n,"Took square root")

let sqrt n=Riter(sqrt n,["Took square root"])

let ~~div2~~ n=~~bind~~(n/~~2.0~~,"Divided by 2")

let div n g=Riter(n/g,[sprintf "Divided by %f" n])

let ~~incr~~ n=~~bind~~(n+~~1.0~~,"Added 1")

let add n g=Riter(n+g,[sprintf "Added %f" n])

let result,log=eval(initV ~~5.0|~~>compose sqrt|>compose ~~incr|~~>compose ~~div2~~)

let result,log=eval((initV>>compose sqrt>>compose(add 1.0)>>compose(div 2.0))5.0)

log|>List.iter(printfn "%s")

printfn "Final value = %f" result

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Initial Value 5.000000

Took square root

Added 1

Added 1.000000

Divided by 2

Divided by 2.000000

Final value = 1.618034

Final value = 0.618034

</pre>

=={{header|Factor}}==

Factor comes with an implementation of Haskell-style monads in the <code>monads</code> vocabulary.