fparsec_practice.fsproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <RootNamespace>fparsec_practice</RootNamespace>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Program.fs" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="FParsec" Version="1.1.1" />
  </ItemGroup>

</Project>

Program.fs

open FParsec

let test (p: Parser<'a, unit>) (str: string) =
    match run p str with
    | Success (result, _, _) -> printfn "Success: %A" result
    | Failure (errorMsg, _, _) -> printfn "Failure: %s" errorMsg

let plus (a: float) (b: float) = a + b

[<EntryPoint>]
let main _ =
    test pfloat "1.23" // Success: 1.23
    test pfloat "a1b2c3" // Failure
    test pfloat "  1.23   " // Failure

    let number = spaces >>. pfloat .>> spaces
    test number "1.23" // Success: 1.23
    test number "  1.23   " // Success: 1.23

    let opPlus =
        spaces >>. stringReturn "plus" plus .>> spaces

    let xPlusY =
        pipe3 number opPlus number (fun x op y -> op x y)

    test xPlusY "1 plus 2" // Success: 3.0

    let manyPlus = chainl1 number opPlus
    test manyPlus "1 plus 2 plus 3 plus 4" // Success: 10.0
    0