Question

In the following ML extract (taken from the Effective ML talk), there is a module abbreviation inside a value binding expression. Is it possible to do the equivalent in F#? I know you can do module abbreviations, I am specifically interested if you can do them “inline” like this.

let command =
  let default_config = { exit_code = 0; message = None } in
  let flags = 
    let module F = Command.Flag in
    [ F.int    "-r" (fun cfg v -> { cfg with exit_code = v });
      F.string "-m" (fun cfg v -> { cfg with message   = v });
    ]
  ...

Answer 1

No, this feature is not available in F#. You can only do top-level module abbreviations (as you say) using:

module F = Command.Flag

You can write these in the middle of a source file, but they have to be at the top-level and their scope is always going to be until the end of a file (or until another definition that hides F). Perhaps if you used this and then had another definition hiding F, it would have similar effect. For example:

module L = List
[0 .. 9] |> L.map ((*) 2) // Uses functional `List.map`

module L = Seq    
[0 .. 9] |> L.map ((*) 2) // Uses lazy `Seq.map`

I agree taht this would be a useful feature in many cases – on the other hand, the F# programming style is sufficiently different from ML, so the advices from Effective ML talk may not directly map to F# programming. If you need to make something a local definition, then the best option would be to define it as an F# object.