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[Caml-list] hash_variant anomalies
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Date: -- (:)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: [Caml-list] hash_variant anomalies
> [Question on tags for constant constructors and constant variant
> constructors]

> This test program seems to indicate otherwise:

No, no, it works as described in the manual, with two caveats:

1- The argument to hash_value() is the name of the variant constructor
without the leading ` (it's a syntactic marker, not part of the
constructor name), hence your test program should be:

> let _ =
>   calc_variant "YES";
>   print_variant YES;
>   calc_variant "NO";
>   print_variant NO;
>   calc_variant "MAYBE";
>   print_variant MAYBE;
>   calc_variant "YES";   <-- removed `
>   print_pvariant `YES;
>   calc_variant "NO";    <-- removed `
>   print_pvariant `NO;
>   calc_variant "MAYBE"; <-- removed `
>   print_pvariant `MAYBE  <-- there was a missing p and a missing `

With these corrections, I get:

variant 'YES' gives 8882703
variant is 1
variant 'NO' gives 34947
variant is 3
variant 'MAYBE' gives 36345617
variant is 5
variant 'YES' gives 8882703
variant is 8882703
variant 'NO' gives 34947
variant is 34947
variant 'MAYBE' gives 36345617
variant is 36345617

2- Owing to the Caml encoding of integers (integer N is represented by
the bit pattern for 2N+1), when the manual says that constant
constructors are numbered starting from 0, it really means that the
constant constructors are represented by the representation of 0, the
representation of 1, etc, that is, by 1, 3, etc.

In contrast, the return value of hash_value() is not a C integer but a
Caml value representing an already encoded integer.

> Also the manual states that both functions "hash_value" and
> "hash_variant" are used to compute the value of variants, but
> mlvalues.h seems to declare only "hash_variant".

Right, that's a typo in the manual: hash_variant() is the correct
function.

- Xavier Leroy
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