Objective Caml 3.00 compatibility issues

Here is a summary of the changes, to help you upgrade. (This is not in the OCaml Changelog, since 2.99 was only a beta)
There are also tools to help you, presented at the end of this note.

Syntax

• Labels
  To keep maximal compatibility with OCaml 2.0x, the syntax of labels in programs changed.
  Here is a conversion table (1):

          label:          ->      ~label:
          :label          ->      ~label
          (label:x : type)->      ~label:(x : type)
          (:label : type) ->      ~(label : type)
          ?label:         ->      ?label:         (unchanged)
          ?:label         ->      ?label
          ?(label:x = v)  ->      ?label:(x = v)
          ?(:label = v)   ->      ?(label = v)
  

  Notice that you cannot use keywords as labels anymore.

  Also, for OLabl programs, the syntax for default values changed:

          ?label:x [< v >] ->     ?label:(x = v)
  

  Semantics of optional parameters were changed between OLabl and OCaml 2.99. See the tutorial for the new semantics.
• Syntax of labels in types do not change (no extra ~).
• Variant types
  In order to get more symmetry with sum types, an extra "of" was introduced before the parameter type.

          [`A int | `B bool]      ->      [`A of int | `B of bool]
  

  Labelling of libraries

  The labellings of many functions in the libraries were changed, for various reasons.
  • To reduce the number of labelled functions in the standard libraries. Basically, labels were omitted wherever they do not provide useful information.

            E.g. List.nth : 'a list -> pos:int -> 'a
            became List.nth : 'a list -> int -> 'a
    

    In practice, most functions of two arguments were removed their labels.
  • To avoid keyword labels, like fun:, to:, in:, end: ...
  There is no exhaustive list of label changes, but here are some of them:

          fun:    ->      f:
          pred:   ->      f:
          end:    ->      stop:
          to:     ->      (no label)
          key:,pos:,len:,size: -> often omitted
  

  • Also there were changes in LablTk, to uniformize, and make easier use in classic mode. The standard parameter order is now
    • [labeled] [optional] [non-labeled] when there are optional arguments
    • [non-labeled] [labeled] when there are no optional arguments
    In the Canvas module, both the widget and the shape id are left unlabelled.

  Helpful tools

  ocaml299to3

  This simple program does label syntax conversion automatically, following table (1). Also, it systematically converts fun: into f:. (Other label changes have to be done by hand.) It is clever enough to distinguish between terms and types (where no conversion should be done). Since it only inserts "~"'s in your code, layout and comments are preserved.

  To convert all files in a directory, just type:

          % ocaml299to3 *.ml
  

  For all modified files, the orignal is kept as file.bak.

  Camlp4 3.00

  If you prefer the old syntax, using only ":", camlp4 can help you. In the upcoming release of camlp4, a grammar pa_olabl is provided that parses Ocaml 2.99 syntax. This does not mean that your programs will run unmodified, since many labels changed in 3.00, but you can keep the old style.

  Quoting Daniel de Raglaudre, here is how to use it:

      To compile with the 2.99 syntax, use
          ocamlc -pp "camlp4o pa_olabl.cmo"
  
      To switch the toplevel syntax to 2.99,
         ocaml -I `camlp4 -where`
         # #load "camlp4o.cma";;    (* Camlp4 parses *)
         # #load "pa_olabl.cmo";;   (* switch parser to 2.99 *)
  
      To create a toplevel parsing the 2.99 syntax,
          ocamlmktop -o olabl -I `camlp4 -where` gramlib.cma \
              camlp4_top.cma pa_olabl.cmo