What does a double exclamation mark (!!) in Fsharp / FAKE?

Question

I've come across the following code and can not understand what operation the double exclamation marks provide. This code-snipet is from a FAKE script used in a CICD system. Microsoft's Symbol and Operator Reference does not list this operator, nor can I find it in FAKE's API Reference.

  !! (projectPackagePath + "/*.zip")
    |> Seq.iter(fun path ->
      trace ("Removing " + path)
      ShellExec tfCommand ("delete " + path + " /noprompt")

Another example of usage

let buildLabelFiles = 
    !!(labelPath @@ "*.txt")

Answer 1

The !! operator takes a file pattern and returns a collection of files matching the pattern.

For example, if you want to print all text files in the current folder, you can write:

for file in !! "*.txt" do
  printfn "%s" file

If you look at the operator definition in the source code, you can see that it is just an alias for creating a IGlobbingPattern value (see the type definition) that includes files given by the specified pattern. The IGlobbingPattern type implements IEnumerable, so you can iterate over the files, but you can do a couple of other things with IGlobbingPattern such as combining two file sets using ++ or removing some files from a file set using --.