Another question on variant types and matching (was: Re: [Caml-list] Question on Variant Types)

[Richard Jones <rich@annexia.org>]

I have another question on variant types which seems to be related to
this, but perhaps the opposite way round.  How can I get the code
below to work?  (It's simplified greatly from a real problem I'm
having).

Rich.

----------------------------------------------------------------------
type colour = [ `Red | `Green | `Blue ]
type colour' = [ colour | `Default ]
type instructions = Set_foreground of colour' | Set_background of colour'

let default_fg : colour = `Red
let default_bg : colour = `Green

let process_instructions =
  List.fold_left (
    fun (fg, bg) ->
      function
      | Set_foreground `Default ->
	  let new_fg = default_fg in
	  (new_fg, bg)
      | Set_foreground new_fg ->
	  (new_fg, bg)
      | Set_background `Default ->
	  let new_bg = default_bg in
	  (fg, new_bg)
      | Set_background new_bg ->
	  (fg, new_bg)
  )

let string_of_colour = function
  | `Red -> "red"
  | `Green -> "green"
  | `Blue -> "blue"

let () =
  let instrs =
    [ Set_foreground `Blue; Set_background `Red; Set_foreground `Default ] in
  let fg, bg = `Green, `Blue in
  let fg, bg = process_instructions (fg, bg) instrs in
  print_endline ("fg = " ^ string_of_colour fg);
  print_endline ("bg = " ^ string_of_colour bg)
----------------------------------------------------------------------

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com