Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Welcome to Software Development on Codidact!

Will you help us build our independent community of developers helping developers? We're small and trying to grow. We welcome questions about all aspects of software development, from design to code to QA and more. Got questions? Got answers? Got code you'd like someone to review? Please join us.

Post History

50%
+0 −0
Q&A How to implement `map` using the fish (>=>, Kleisli composition) operator in F#?

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 answer  ·  posted 8mo ago by toraritte‭  ·  last activity 8mo ago by r~~‭

#1: Initial revision by user avatar toraritte‭ · 2024-04-02T17:18:35Z (8 months ago)
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)
```