Polymorphic variant to string?

Polymorphic variant to string?

[Chris Campbell]

Hi,

Is there a general method for converting polymorphic variants to
strings?  The method using camlp4 assumes the source is compiled with
it, which is not the case.  Basically I want to print a list of
variants if a unit tests fail, so we can see exactly the contents of
the list at that point.

The polymorphic variant type resides in a library and is used by a
module in my program.  I need to check the values returned match which
is easy, except when the code tested is erroneous and I want to see
what's going on.

Any ideas?  Suggestions?


Cheers,
Chris

Re: [Caml-list] Polymorphic variant to string?

[Jacques Garrigue]

From: Chris Campbell <cyberdanx@gmail.com>

> Is there a general method for converting polymorphic variants to
> strings?  The method using camlp4 assumes the source is compiled with
> it, which is not the case.  Basically I want to print a list of
> variants if a unit tests fail, so we can see exactly the contents of
> the list at that point.

The internal representation of a polymorphic variant tag is a hash by
a given function (you can find it in typing/btype.ml)
So, if you have the list of all possible variant tags, and their hash
values, you just have to do a reverse lookup.
Note that in theory, you need a list by type, as some independent
types might map different tags to the same hash value, but in practice
this should not be a problem: the probability of conflict is low
enough.
And of course this only apply to variant tags, not to argument values.

Jacques Garrigue

Re: [Caml-list] Polymorphic variant to string?

[Richard Jones]

On Wed, Nov 16, 2005 at 06:44:34PM +0000, Chris Campbell wrote:
> Is there a general method for converting polymorphic variants to
> strings?  The method using camlp4 assumes the source is compiled with
> it, which is not the case.  Basically I want to print a list of
> variants if a unit tests fail, so we can see exactly the contents of
> the list at that point.

You might want to try Std.dump in extlib.  It only prints out the
hash, but you can precalculate those given a list of variant names.

rich@oirase:/tmp$ cat test.ml
print_endline (Std.dump [ `Foo; `Bar; `Baz 123.0 ])
rich@oirase:/tmp$ ocamlfind ocamlc -package extlib test.ml -linkpkg -o test
rich@oirase:/tmp$ ./test
[3505894; 3303859; (3303867, 123.)]

Rich.

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