FSharpPlus

Introducing FSharpPlus

• Download binaries from Nuget, use the latest CI version.

• Open an F# script file or the F# interactive, reference the library and open the namespace

#r @"nuget: FSharpPlus"

Now we'll start with a quick overview of the features presented in F#+.

Generic functions

They are automatically available when opening the FSharpPlus namespace

here's an example with map (fmap for Haskellers, Select for C-sharpers):

  open FSharpPlus

  map string [|2;3;4;5|]
  // val it : string [] = [|"2"; "3"; "4"; "5"|]

  map ((+) 9) (Some 3)
  // val it : int option = Some 12

  open FSharpPlus.Data

  map string (NonEmptyList.create 2 [3;4;5])
  // val it : NonEmptyList<string> = {Head = "2"; Tail = ["3"; "4"; "5"];}

They're also available for your own types as long as they contain the appropriated method with the expected signature

  type Tree<'t> =
      | Tree of 't * Tree<'t> * Tree<'t>
      | Leaf of 't
      static member Map (x:Tree<'a>, f) = 
          let rec loop f = function
              | Leaf x -> Leaf (f x)
              | Tree (x, t1, t2) -> Tree (f x, loop f t1, loop f t2)
          loop f x
  
  map ((*) 10) (Tree(6, Tree(2, Leaf 1, Leaf 3), Leaf 9))
  // val it : Tree<int> = Tree (60,Tree (20,Leaf 10,Leaf 30),Leaf 90)

Generic functions may be seen as an exotic thing in F# that only saves a few key strokes (map instead of List.map or Array.map) still they allow you to reach a higher abstraction level, using ad-hoc polymorphism.

But more interesting is the use of operators. You can't prefix them with the module they belong to, well you can but then it's no longer an operator. As an example many F# libraries define the bind operator (>>=) but it's not generic so if you use two different types which are both monads you will need to prefix it e.g. State.(>>=) and Reader.(>>=) which defeats the purpose of having an operator.

Here you have a ready-to-use generic bind operator: >>=

  let x = ["hello";" ";"world"] >>= (fun x -> Seq.toList x)
  // val x : char list = ['h'; 'e'; 'l'; 'l'; 'o'; ' '; 'w'; 'o'; 'r'; 'l'; 'd']


  let tryParseInt : string -> int option = tryParse
  let tryDivide x n = if n = 0 then None else Some (x / n)

  let y = Some "20" >>= tryParseInt >>= tryDivide 100
  // val y : int option = Some 5

You have also the Kleisli composition (fish) operator: >=>

Which is becoming popular in F# after the Railway Oriented Programming tutorial series

  let parseAndDivide100By = tryParseInt >=> tryDivide 100

  let parsedAndDivide100By20 = parseAndDivide100By "20"   // Some 5
  let parsedAndDivide100By0' = parseAndDivide100By "zero" // None
  let parsedAndDivide100By0  = parseAndDivide100By "0"    // None

  let parseElement n = List.tryItem n >=> tryParseInt
  let parsedElement  = parseElement 2 ["0"; "1";"2"]

But don't forget the above used operators are generic, so we can change the type of our functions and we get a different functionality for free:

The test code remains unchanged, but we get a more interesting functionality

  let parseAndDivide100By = tryParseInt >=> tryDivide 100

  let parsedAndDivide100By20 = parseAndDivide100By "20"   // Choice1Of2 5
  let parsedAndDivide100By0' = parseAndDivide100By "zero" // Choice2Of2 "Failed to parse zero"
  let parsedAndDivide100By0  = parseAndDivide100By "0"    // Choice2Of2 "Can't divide by zero"

Also when working with combinators, the generic applicative functor (space invaders) operator is very handy: <*>

  let sumAllOptions = Some (+) <*> Some 2 <*> Some 10     // val sumAllOptions : int option = Some 12

  let sumAllElemets = [(+)] <*> [10; 100] <*> [1; 2; 3]   // int list = [11; 12; 13; 101; 102; 103]

For more details and features, see generic operators and functions

Here are all generic operators and functions

And here's a short explanation of Functor, Applicative and Monad abstractions with code samples.

Lens

from https://github.com/ekmett/lens/wiki/Examples

First, open F#+ Lens

  open FSharpPlus.Lens

Now, you can read from lenses (_2 is a lens for the second component of a tuple)

  let r1 = ("hello","world")^._2
  // val it : string = "world"

and you can write to lenses.

  let r2 = setl _2 42 ("hello","world")
  // val it : string * int = ("hello", 42)

Composing lenses for reading (or writing) goes in the order an imperative programmer would expect, and just uses (<<).

  let r3 = ("hello",("world","!!!"))^.(_2 << _1)
  // val it : string = "world"

  let r4 = setl (_2 << _1) 42 ("hello",("world","!!!"))
  // val it : string * (int * string) = ("hello", (42, "!!!"))

You can make a Getter out of a pure function with to'.

  let r5 = "hello"^.to' length
  // val it : int = 5

You can easily compose a Getter with a Lens just using (<<). No explicit coercion is necessary.

  let r6 = ("hello",("world","!!!"))^. (_2 << _2 << to' length)
  // val it : int = 3

As we saw above, you can write to lenses and these writes can change the type of the container. (.->) is an infix alias for set.

  let r7 = _1 .-> "hello" <| ((),"world")
  // val it : string * string = ("hello", "world")

It can be used in conjunction with (|>) for familiar von Neumann style assignment syntax:

  let r8 = ((), "world") |> _1 .-> "hello"
  // val it : string * string = ("hello", "world")

Conversely view, can be used as an prefix alias for (^.).

  let r9 = view _2 (10,20)
  // val it : int = 20

For more details:

Here's a full tour of lens and all other optics

Have a look at all lens functions