Re: [Caml-list] Pattern matching question

[Seth J. Fogarty]

On 7/19/06, Jim Miller <gordon....@gmail.com> wrote:
> I'm trying to use an OCaml program to form an umambiguous protocol
> specification. I've convinced myself that by proper definition of my types,
> and avoiding using the _ in matches, I can get OCaml to help me verify that
> I have a complete specification. I'm doing this by defining a function that
> specifies the behavior of one of the protocol partners when it receives a
> message. Here is an example of what I have.
>
> let requestOp = REQUEST_KEY | SUCCESS;;
>
> let message = None | Register of requestOp * int;;
>
> let receive idList msg =
> match message with
> None -> None
>
> | Register( REQUEST_KEY, id ) -> begin
> match idList with
> [] -> Register(SUCCESS, id)
> | l when (List.mem id l) -> Register(Success, (next_available_id
> l))
> | _ -> Register(Success, id)
> ;;

Try something like

match idList with
[] -> Register(SUCCESS, id)
| l -> if List.mem id l then Register(foo) else Register(bar)

And replace the guard with an if statement.

--
Seth Fogarty sfogarty@[gmail.com|rice.edu|livejournal]
Neep-neep at large AIM: Sorrath
"I know there are people in this world who do not love their fellow
human beings - and I hate people like that" --Tom Lehrer.

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[Luc Maranget]

> I'm trying to use an OCaml program to form an umambiguous protocol
> specification. I've convinced myself that by proper definition of my types,
> and avoiding using the _ in matches, I can get OCaml to help me verify that
> I have a complete specification. I'm doing this by defining a function that
> specifies the behavior of one of the protocol partners when it receives a
> message. Here is an example of what I have.
>
> let requestOp = REQUEST_KEY | SUCCESS;;
>
> let message = None | Register of requestOp * int;;
>
> let receive idList msg =
> match message with
> None -> None
>
> | Register( REQUEST_KEY, id ) -> begin
> match idList with
> [] -> Register(SUCCESS, id)
> | l when (List.mem id l) -> Register(Success, (next_available_id
> l))
> | _ -> Register(Success, id)
> ;;
>

To avoid | _ -> ... matching clauses, have a look at the -w E warning.
<http://caml.inria.fr/pub/docs/manual-ocaml/manual022.html>
Section 8.2 'Options'

I cannot get your exact point, besides the above code does not even go through
parsing (missing end), let -> type etc.

In fact what you want to achieve is still unclear.
As regards your message and requestOp types there should be no problem,

Something like

match msg with
| None -> ...
| Register (REQUEST_KEY, id) -> ...
| Register (SUCCESS, id) -> ...

Is easily shown to be exhaustive.

As regards your list example, perhaps you'd like better

match idList with
| [] -> Register(SUCCESS, id)

| _::_ ->
if List.mem id idList then Register(SUCCESS, next_available idList)
else Register(SUCCESS, id)

or even

match idList with
| [] -> Register(SUCCESS, id)

| _::_ when List.mem id idList -> Register(SUCCESS, next_available idList)
| _::_ -> Register(SUCCESS, id)

Both are easily shown to be exhaustive, but frankly, this brings you
no more guarantee than the much more direct code

if List.mem id idList then
Register(SUCCESS, next_available_id idList))
else
Register (SUCESS, id)

--
Luc Maranget