Add YarnHelper that works the same as NpmHelper but calls yarn

src/app/FakeLib/FakeLib.fsproj

@@ -62,6 +62,7 @@
     <Compile Include="PsExecHelper.fs" />
     <Compile Include="NpmHelper.fs" />
     <Compile Include="BowerHelper.fs" />
+    <Compile Include="YarnHelper.fs" />
     <Compile Include="AppVeyor.fs" />
     <Compile Include="BitbucketPipelines.fs" />
     <Compile Include="TaskRunnerHelper.fs" />
@@ -531,4 +532,4 @@
       </ItemGroup>
     </When>
   </Choose>
-</Project>
+</Project>

src/app/FakeLib/YarnHelper.fs

@@ -0,0 +1,115 @@
+/// Contains function to run yarn tasks
+module Fake.YarnHelper
+open Fake
+open System
+open System.IO
+open System.Diagnostics
+
+/// Default paths to Yarn
+let private yarnFileName =
+    match isWindows with
+    | true ->
+        System.Environment.GetEnvironmentVariable("PATH")
+        |> fun path -> path.Split ';'
+        |> Seq.tryFind (fun p -> p.IndexOf("yarn", StringComparison.OrdinalIgnoreCase) >= 0)
+        |> fun res ->
+            match res with
+            | Some yarn when File.Exists (sprintf @"%s\yarn.cmd" yarn) -> (sprintf @"%s\yarn.cmd" yarn)
+            | _ -> "./packages/Yarnpkg.js/tools/yarn.cmd"
+    | _ ->
+        let info = new ProcessStartInfo("which","yarn")
+        info.StandardOutputEncoding <- System.Text.Encoding.UTF8
+        info.RedirectStandardOutput <- true
+        info.UseShellExecute        <- false
+        info.CreateNoWindow         <- true
+        use proc = Process.Start info
+        proc.WaitForExit()
+        match proc.ExitCode with
+            | 0 when not proc.StandardOutput.EndOfStream ->
+              proc.StandardOutput.ReadLine()
+            | _ -> "/usr/bin/yarn"
+
+/// Arguments for the Yarn install command
+type InstallArgs =
+| Standard
+| Flat
+| Force
+| Har
+| NoLockFile
+| Production
+| PureLockfile
+
+/// The list of supported Yarn commands. The `Custom` alternative
+/// can be used for other commands not in the list until they are
+/// implemented
+type YarnCommand =
+| Install of InstallArgs
+| Add of string
+| Custom of string
+
+/// The Yarn parameter type
+[<CLIMutable>]
+type YarnParams =
+    { Src: string
+      YarnFilePath: string
+      WorkingDirectory: string
+      Command: YarnCommand
+      Timeout: TimeSpan }
+
+/// Yarn default parameters
+let defaultYarnParams =
+    { Src = ""
+      YarnFilePath = yarnFileName
+      Command = Install Standard
+      WorkingDirectory = "."
+      Timeout = TimeSpan.MaxValue }
+
+let private parseInstallArgs = function
+    | Standard -> ""
+    | Flat -> " --flat"
+    | Force -> " --force"
+    | Har -> " --har"
+    | NoLockFile -> " --no-lockfile"
+    | Production -> " --production"
+    | PureLockFile -> " --pure-lockfile"
+
+let private parse = function
+    | Install installArgs -> sprintf "install%s" (installArgs |> parseInstallArgs)
+    | Add str -> sprintf "add %s" str
+    | Custom str -> str
+
+let run yarnParams =
+    let yarnPath = Path.GetFullPath(yarnParams.YarnFilePath)
+    let arguments = yarnParams.Command |> parse
+    let ok =
+        execProcess (fun info ->
+            info.FileName <- yarnPath
+            info.WorkingDirectory <- yarnParams.WorkingDirectory
+            info.Arguments <- arguments) yarnParams.Timeout
+    if not ok then failwith (sprintf "'yarn %s' task failed" arguments)
+
+/// Runs yarn with the given modification function. Make sure to have yarn installed,
+/// you can install yarn with nuget or a regular install. To change which `Yarn` executable
+/// to use you can set the `YarnFilePath` parameter with the `setParams` function.
+///
+/// ## Parameters
+///
+/// - `setParams` - Function used to overwrite the Yarn default parameters.
+///
+/// ## Sample
+///
+///         Target "Web" (fun _ ->
+///             Yarn (fun p ->
+///                     { p with
+///                         Command = Install Standard
+///                         WorkingDirectory = "./src/FAKESimple.Web/"
+///                     })
+///
+///             Yarn (fun p ->
+///                     { p with
+///                         Command = (Run "build")
+///                         WorkingDirectory = "./src/FAKESimple.Web/"
+///                     })
+///         )
+let Yarn setParams =
+    defaultYarnParams |> setParams |> run