Install Fantomas and add check to build pipeline

.config/dotnet-tools.json

@@ -27,4 +27,4 @@
       ]
     }
   }
-}
+}

build.fsx

@@ -165,9 +165,40 @@ Target.create "Help" (fun _ ->
     printfn ""
 )
 
+let sourceFiles = 
+    !! "src/**/*.fs" ++ "src/**/*.fsi" ++ "build.fsx"
+    -- "src/**/obj/**/*.fs"
+    -- "src/AssemblyInfo*.fs"
+
+Target.create "Format" (fun _ ->
+    let result =
+        sourceFiles
+        |> Seq.map (sprintf "\"%s\"")
+        |> String.concat " "
+        |> DotNet.exec id "fantomas"
+
+    if not result.OK then
+        printfn "Errors while formatting all files: %A" result.Messages)
+
+Target.create "CheckFormat" (fun _ ->
+    let result =
+        sourceFiles
+        |> Seq.map (sprintf "\"%s\"")
+        |> String.concat " "
+        |> sprintf "%s --check"
+        |> DotNet.exec id "fantomas"
+
+    if result.ExitCode = 0 then
+        Trace.log "No files need formatting"
+    elif result.ExitCode = 99 then
+        failwith "Some files need formatting, run `dotnet fake build -t Format` to format them"
+    else
+        Trace.logf "Errors while formatting: %A" result.Errors
+        failwith "Unknown errors while formatting")
+
 Target.create "All" ignore
 
-"Clean" ==> "AssemblyInfo" ==> "Build"
+"Clean" ==> "AssemblyInfo" ==> "CheckFormat" ==> "Build"
 "Build" ==> "CleanDocs" ==> "GenerateDocs" ==> "All"
 "Build" ==> "NuGet" ==> "All"
 "Build" ==> "All"