Two camlp4 questions

Two camlp4 questions

[Richard Jones]

(1) How do I match on the pattern which is literally '_' in the
original code?

    match mypatt with
    | <:patt< _ >> -> ...

seems like it matches any pattern.

(2) Is there a function hiding anywhere which tests whether a pattern
is exhaustive?  Here's the problem I have: I want to generate code
like this:

    <:expr< match $someexpr$ with $mypatt$ -> $code$ | _ -> () >>

However this gives a compile-time warning if mypatt is already
exhaustive because the second case could never be matched.  If mypatt
is already exhaustive then I'd want to generate this code instead to
avoid the warning:

    <:expr< match $someexpr$ with $mypatt$ -> $code$ >>

I hacked around it a little with this function:

  let pattern_is_exhaustive = function
  | <:patt< $lid:_$ >> -> true
  | _ -> false

but I guess you can see that this function is not a complete solution.

Rich.

-- 
Richard Jones
Red Hat

Re: [Caml-list] Two camlp4 questions

[Martin Jambon]

On Fri, 25 Apr 2008, Richard Jones wrote:

> (1) How do I match on the pattern which is literally '_' in the
> original code?
>
>    match mypatt with
>    | <:patt< _ >> -> ...
>
> seems like it matches any pattern.

Strange.


> (2) Is there a function hiding anywhere which tests whether a pattern
> is exhaustive?

It's something that can't be done with camlp4.
If in some module M, some type t is defined by "type t = A | B | C"
then knowing that "M.A -> 1 | M.B -> 2" is incomplete requires access to 
more information than camlp4 has.


>  Here's the problem I have: I want to generate code
> like this:
>
>    <:expr< match $someexpr$ with $mypatt$ -> $code$ | _ -> () >>
>
> However this gives a compile-time warning if mypatt is already
> exhaustive because the second case could never be matched.  If mypatt
> is already exhaustive then I'd want to generate this code instead to
> avoid the warning:
>
>    <:expr< match $someexpr$ with $mypatt$ -> $code$ >>

One possible hack is to add "when true" guards everywhere and one final 
"| _ -> assert false" case.
This disables completeness checking and turns off warnings, at your own 
risk.

Note: "assert false" is not ideal, since you want to raise Match_failure 
with the proper location. <:expr< match () with [] >> should do it 
(assuming quotations in the revised syntax)


> I hacked around it a little with this function:
>
>  let pattern_is_exhaustive = function
>  | <:patt< $lid:_$ >> -> true
>  | _ -> false
>
> but I guess you can see that this function is not a complete solution.
>
> Rich.
>
> -- 
> Richard Jones
> Red Hat
>
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Re: [Caml-list] Two camlp4 questions

[Richard Jones]

On Fri, Apr 25, 2008 at 03:58:17PM +0200, Martin Jambon wrote:
> One possible hack is to add "when true" guards everywhere and one final 
> "| _ -> assert false" case.
> This disables completeness checking and turns off warnings, at your own 
> risk.

Nice, thanks.  In this case I don't want the exhaustiveness checks at
all so this works out fine for me.

Rich.

-- 
Richard Jones
Red Hat

Re: [Caml-list] Two camlp4 questions

[Nicolas Pouillard]

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Excerpts from Richard Jones's message of Fri Apr 25 15:18:12 +0200 2008:
> (1) How do I match on the pattern which is literally '_' in the
> original code?
> 
>     match mypatt with
>     | <:patt< _ >> -> ...
> 
> seems like it matches any pattern.

I don't think so, it's supposed to match only the wildcard pattern.

Proof:
  $ camlp4oof -str 'match x with <:patt< _ >> -> y'
  match x with | Ast.PaAny _ -> y

-- 
Nicolas Pouillard aka Ertai

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Re: [Caml-list] Two camlp4 questions

[Richard Jones]

On Sat, Apr 26, 2008 at 02:54:12PM +0200, Nicolas Pouillard wrote:
> Excerpts from Richard Jones's message of Fri Apr 25 15:18:12 +0200 2008:
> > (1) How do I match on the pattern which is literally '_' in the
> > original code?
> > 
> >     match mypatt with
> >     | <:patt< _ >> -> ...
> > 
> > seems like it matches any pattern.
> 
> I don't think so, it's supposed to match only the wildcard pattern.
> 
> Proof:
>   $ camlp4oof -str 'match x with <:patt< _ >> -> y'
>   match x with | Ast.PaAny _ -> y

OK thanks - I was obviously confused.

Rich.

-- 
Richard Jones
Red Hat