add basic npm support

src/app/FakeLib/FakeLib.fsproj

@@ -55,6 +55,7 @@
     <Compile Include="ServiceControllerHelper.fs" />
     <Compile Include="GuardedAwaitObservable.fs" />
     <Compile Include="ProcessHelper.fs" />
+    <Compile Include="NpmHelper.fs" />
     <Compile Include="AppVeyor.fs" />
     <Compile Include="TaskRunnerHelper.fs" />
     <Compile Include="Globbing\Globbing.fs" />

src/app/FakeLib/NpmHelper.fs

@@ -0,0 +1,85 @@
+/// Contains function to run npm tasks
+module Fake.NpmHelper
+open Fake
+open System
+open System.IO
+
+/// Default paths to Npm
+let private npmFileName =
+    match isUnix with
+    | true -> "/usr/local/bin/npm"
+    | _ -> "./packages/Npm.js/tools/npm.cmd"
+
+/// Arguments for the Npm install command
+type InstallArgs =
+| Standard
+| Forced
+
+/// The list of supported Npm commands. The `Custom` alternative
+/// can be used for other commands not in the list until they are
+/// implemented
+type NpmCommand =
+| Install of InstallArgs
+| Run of string
+| Custom of string
+
+/// The Npm parameter type
+type NpmParams = 
+    { Src: string
+      NpmFilePath: string
+      WorkingDirectory: string
+      Command: NpmCommand
+      Timeout: TimeSpan }
+
+/// Npm default parameters
+let defaultNpmParams = 
+    { Src = ""
+      NpmFilePath = npmFileName
+      Command = Install Standard
+      WorkingDirectory = "."
+      Timeout = TimeSpan.MaxValue }
+
+let private parseInstallArgs = function
+    | Standard -> ""
+    | Forced -> " --force"
+
+let private parse = function
+    | Install installArgs -> sprintf "install%s" (installArgs |> parseInstallArgs)
+    | Run str -> sprintf "run %s" str
+    | Custom str -> str
+
+let run npmParams =
+    let npmPath = Path.GetFullPath(npmParams.NpmFilePath)
+    let arguments = npmParams.Command |> parse
+    let ok = 
+        execProcess (fun info ->
+            info.FileName <- npmPath
+            info.WorkingDirectory <- npmParams.WorkingDirectory
+            info.Arguments <- arguments) npmParams.Timeout
+    if not ok then failwith (sprintf "'npm %s' task failed" arguments)
+
+/// Runs npm with the given modification function. Make sure to have npm installed,
+/// you can install npm with nuget or a regular install. To change which `Npm` executable
+/// to use you can set the `NpmFilePath` parameter with the `setParams` function.
+///
+/// ## Parameters
+///
+///  - `setParams` - Function used to overwrite the Npm default parameters.
+///
+/// ## Sample
+///
+///        Target "Web" (fun _ ->
+///            Npm (fun p ->
+///                   { p with
+///                       Command = Install Standard
+///                       WorkingDirectory = "./src/FAKESimple.Web/"
+///                   })
+///
+///            Npm (fun p ->
+///                   { p with
+///                       Command = (Run "build")
+///                       WorkingDirectory = "./src/FAKESimple.Web/"
+///                   })
+///        )
+let Npm setParams =
+    defaultNpmParams |> setParams |> run