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This type is an implementation of the [Free Monad](https://www.google.com/search?q=free+monad) which is generic to any [Functor](abstraction-functor.html).
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The Free Monad is used typically to describe a pure program at high level and separately write different interpreters for it.
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Related Types
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-------------
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- [Coproduct](type-coproduct.html): A [Functor](abstraction-functor.html) used in conjunction with the Free Monad to combine different instruction sets.
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Examples
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--------
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Free monad-interpreter in F# from [Mark Seemann's blog](https://blog.ploeh.dk/2017/07/17/a-pure-command-line-wizard) but encoded with Free.
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*)
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(**
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```f#
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#r @"nuget: FSharpPlus"
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```
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*)
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-
openFSharpPlus
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openSystem
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openFSharpPlus
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openFSharpPlus.Data
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typeCommandLineInstruction<'t>=
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| ReadLine of(string->'t)
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| WriteLine ofstring*'t
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withstatic memberMap(x,f)=
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match x with
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| ReadLine g -> ReadLine (f << g)
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| WriteLine (s, g)-> WriteLine (s, f g)
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letreadLine= Free.liftF (ReadLine id)
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letwriteLine s = Free.liftF (WriteLine (s,()))
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let recinterpretCommandLine= Free.run >>function
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| Pure x -> x
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| Roll (ReadLine next)-> Console.ReadLine ()|> next |> interpretCommandLine
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| Roll (WriteLine (s, next))->
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Console.WriteLine s
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next |> interpretCommandLine
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let recreadQuantity= monad {
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do! writeLine "Please enter number of diners:"
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let!l= readLine
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match tryParse l with
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| Some dinerCount ->return dinerCount
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| None ->
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do! writeLine "Not an integer."
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return! readQuantity }
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let recreadDate= monad {
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do! writeLine "Please enter your desired date:"
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let!l= readLine
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match DateTimeOffset.TryParse l with
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|true, dt ->return dt
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|_->
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do! writeLine "Not a date."
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return! readDate }
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letreadName= monad {
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do! writeLine "Please enter your name:"
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return! readLine }
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letreadEmail= monad {
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do! writeLine "Please enter your email address:"
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return! readLine }
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typeReservation={
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Date :DateTimeOffset
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Name :string
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Email :string
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Quantity :int }
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withstatic memberCreate(Quantity,Date,Name,Email)={ Date = Date; Name = Name; Email = Email; Quantity = Quantity }
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letreadReservationRequest=
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curryN Reservation.Create
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<!> readQuantity
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<*> readDate
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<*> readName
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<*> readEmail
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letmainFunc()=
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readReservationRequest
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>>=(writeLine <<(sprintf "%A"))
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|> interpretCommandLine
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(**
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More reading
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------------
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- Highly recommended Matt Thornton's blog [Grokking Free monads](https://dev.to/choc13/grokking-free-monads-9jd) and [Interpreting Free Monads](https://dev.to/choc13/interpreting-free-monads-3l3e).
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It contains examples using F#+ and an explanation from scratch.
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- Mark Seemann's blog has an [article series](https://blog.ploeh.dk/2017/06/27/pure-times/) which ends
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up describing Free Monads although he doesn't use F#+ and therefore either repeats boilerplate code or switches to Haskell.
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Anyways some code from those series (like the above fragment) can be found in [our test suite for Free](https://github.com/fsprojects/FSharpPlus/blob/master/tests/FSharpPlus.Tests/Free.fs) simplified using Free and Coproduct types.
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- Scott Wlaschin's [13 ways of looking at a turtle](https://fsharpforfunandprofit.com/posts/13-ways-of-looking-at-a-turtle) is also a series which ends up defining a Free Monad, without using F#+ but with boilerplate code instead.
Copy file name to clipboardExpand all lines: src/FSharpPlus/Control/Applicative.fs
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@@ -54,6 +54,23 @@ type Apply =
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|true, vx -> dct.Add (k, vf vx)
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|_->()
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dct
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static member ``<*>`` (f:IDictionary<'Key,_>,x:IDictionary<'Key,'T>,[<Optional>]_output:IDictionary<'Key,'U>,[<Optional>]_mthd:Apply):IDictionary<'Key,'U>=
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letdct= Dictionary ()
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for KeyValue(k, vf)in f do
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match x.TryGetValue k with
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|true, vx -> dct.Add (k, vf vx)
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|_->()
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dct :> IDictionary<'Key,'U>
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static member ``<*>`` (f:IReadOnlyDictionary<'Key,_>,x:IReadOnlyDictionary<'Key,'T>,[<Optional>]_output:IReadOnlyDictionary<'Key,'U>,[<Optional>]_mthd:Apply):IReadOnlyDictionary<'Key,'U>=
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letdct= Dictionary ()
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for KeyValue(k, vf)in f do
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match x.TryGetValue k with
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|true, vx -> dct.Add (k, vf vx)
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|_->()
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dct :> IReadOnlyDictionary<'Key,'U>
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#if!FABLE_COMPILER
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static member ``<*>`` (f:Expr<'T->'U>,x:Expr<'T>,[<Optional>]_output:Expr<'U>,[<Optional>]_mthd:Apply)= Expr.Cast<'U>(Expr.Application (f, x))
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