Re: variant filtering

From: Jacques Garrigue (garrigue@kurims.kyoto-u.ac.jp)
Date: Fri Feb 18 2000 - 15:41:29 MET

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    From: Manuel Fahndrich <maf@microsoft.com>

    > Variants are great, but pattern matching should filter used cases in the
    > default branch. This seems to be a dual of the record modification problem
    > we've seen some time ago on this list.
    >
    > Consider the following code:
    >
    >
    > type 'a status = [`Success 'a | `FailureA | `FailureB | `FailureC ];;
    >
    > let f () = (`Success "Yeah" :> string status)
    >
    >
    > let g () =
    > match f () with
    > `Success s -> `Success 1
    > | other -> (other :> int status)

    Indeed the above code will cause a type error.
    However, with variants there is a workaround:

    let g () =
      match f () with
        `Success s -> `Success 1
      | `FailureA | `FailureB | `FailureC as other -> other

    Compared to Pottier's Wallace (or Pessaux's exceptions), you have to
    explicitely give the list of the cases matched by other. Still this
    will produce the same code as your example would have (had it be
    typable).

    Originally, this was a design choice to avoid having to handle
    specifically row variables: now you cannot name them, but only the
    variant type containing them. This means that two different variant
    types cannot share the same row variable, as would be needed in your
    example.

    With the conjunctive typing in ocaml 2.99 variants, there may also be
    new theoretical problems for an approach where rows can be shared. I
    did not really check.

    Jacques
    ---------------------------------------------------------------------------
    Jacques Garrigue Kyoto University garrigue at kurims.kyoto-u.ac.jp
                    <A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>



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