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I'm learning monadic composition through Scott Wlaschin's Railway-oriented Programming post. Oncebind, switch, and >=> functions are defined, he introduces map to show how to "turn a one-trac...
#1: Initial revision
How to implement `map` using the fish (>=>, Kleisli composition) operator in F#?
I'm learning monadic composition through Scott Wlaschin's [Railway-oriented Programming](https://fsharpforfunandprofit.com/posts/recipe-part2/) post. Once`bind`, `switch`, and `>=>` functions are defined, he introduces `map` to show how to "_turn a one-track function into a two-track function_". That is: ``` f: a -> b => f': T<a,c> -> T<b,c> ``` The implementation in the article is the following: ``` let map oneTrackFunction twoTrackInput = match twoTrackInput with | Success s -> Success (oneTrackFunction s) | Failure f -> Failure f ``` Did an an equivalent implementation using `switch` and `bind` as an exercise, ``` let map' f = bind (switch f) ``` but when I tried to implement `map` with `>=>`, I arrived at this ugly mess: ``` let map'' f result = match result with | Ok o -> ((fun _ -> result) >=> (switch f)) o | Error e -> Error e ``` <sup>**Note to self**: `o` could be any value of type `'a` (if `result : Result<'a,'c>`), because `f`'s input is already saved in the closure used as `>=>`'s first operand, but this was the only way I could think of to keep it more generic.</sup> Is there a "cleaner" implementation similar to `map'`s? --- ### Notes I used the following example to test the `map`s above: ``` map ((+) 2) ((Ok 27) : Result<int,string>) ``` Used implementations of `bind`, `switch`, `>=>`: ``` let bind ( f : 'a -> Result<'b,'c>) (result : Result<'a,'c>) = match result with | Ok o -> f o | Error e -> Error e let switch (f : 'a -> 'b) (x : 'a ) = f x |> Ok let (>=>) (f : 'a -> Result<'b,'error>) (g : 'b -> Result<'c,'error>) = f >> (bind g) ```