FileSetCE.fs

type FileSetBuilder() =
    member x.Delay f = f
    member x.Run f = f() |> Scan
    member x.Yield (()) = { Include "" with Includes = [] }
    
    [<CustomOperation ("take", MaintainsVariableSpace = true)>]
    member x.Take (fs, pattern: string) = fs ++ pattern

    [<CustomOperation ("notTake", MaintainsVariableSpace = true)>]
    member x.NotTake (fs: FileIncludes, pattern: string) = fs -- pattern

let files = FileSetBuilder()

Target "Build" {
    files { 
        take "**\bin\**\*Tests.dll"
        take "**\bin2\**\*Tests.dll"
        take "**\binaries\**\*"
        notTake "**\bin2\Special*\*"
        take "**\binaries2\**\*"
    }
    |> MSBuildDebug @".\out1" "Build"
    |> Log "AppBuild-Output: " 
}

// old syntax
Target "Build" <| fun _ ->
    !+ "**\bin\**\*Tests.dll"
    ++ "**\bin2\**\*Tests.dll"
    ++ "**\binaries\**\*"
    -- "**\bin2\Special*\*"
    ++ "**\binaries2\**\*"
    |> Scan
    |> MSBuildDebug @".\out1" "Build"
    |> Log "AppBuild-Output: " 

I for one would have the least surprise when obvious names are used with the builtin function composition options (like pipe operator). I feel quite safe with the scan/include/exclude syntax. It's nice to have a more 'obvious' syntax for the beginner as well as a more 'terse' syntax for the advanced users.

I'm rather sceptical about C# fluent style these days. However, as an option among others it could be ideal for using by C# devs.