[m-rev.] for review: fix unused pred warnings for library/store.m

[Julien Fischer]

On Sat, 21 Nov 2020, Zoltan Somogyi wrote:

> For review by anyone. The diff is with -b.

> Fix unused pred warnings in library/store.m.
> 
> When library/store.m was compiled with LIBARY_INTERMODULE = no,

s/LIBARY/LIBRARY/

> it used to get these warnings:
>
>     store.m:264: Warning: predicate `store_equal'/2 mode 0 is never called.
>     store.m:270: Warning: predicate `store_compare'/3 mode 0 is never called.
> 
> The reason was that
> 
> - the only places that refer to these predicates are the "where equality is"
>   and "where comparison is" clauses of the foreign type definitions
>   for the store/1 type constructor,
> 
> - when the compiler constructs the unify and compare predicates for store/1,
>   it is supposed to make them call store_equal and store_compare respectively,
> 
> - BUT store/1 is listed as a builtin dummy type, along with io.state.
>   They are indeed dummy types in the sense that they don't need any
>   representation, but they are NOT dummy types in the sense that
>   they have one function symbol which is a constant.
> 
> - Nevertheless, because of this, the unify and compare predicates for store/1
>   used to get a clause body that always succeeds, since that is what is
>   appropriate for a type that has one function symbol which is a constant.
> 
> For I/O states, this does not matter, since the in,in mode of the arguments
> of the unify and compare predicates means that any I/O state being compared
> cannot be used either as a di input or as a uo output, so such erroneous
> comparisons would always have been detected, though the error message
> would have been misleading. For stores, any attempts to unify or compare them
> would have silently succeeded. Even though store_equal and store_compare
> were designed to abort the program in such an event, the old, wrong code in
> unify_proc.m meant that they were never called.
> 
> This meant that store_equal and store_compare were never called, but with
> intermodule optimization turned on (the default), the presence of
> opt_exported items such as instances would have suppressed such warnings.

Revise the wording of that paragraph, I don't know what you mean by instances
there.  (The reason the warnings would be suppressed is due to the foreign_type
pragmas being opt_exported.)

> 
> library/io.m:
>     Specify clones of store_equal and store_compare as the type-specific
>     unify and compare predicates for I/O states.
> 
> compiler/unify_proc.m:
>     Now that both builtin dummy type ctors have type-specific unify
>     and compare predicates that always abort, construct their overall
>     unidy and compare predicates from them; do not special case them.

s/unidy/unify/

>
>     Note a potential problem with applying an equality pre-test in
>     user-specified unify and compare predicates that always abort.
>     This includes io.state/0 and store.store/1, but may also include
>     other type constructors.

The compiler could check for (and reject) direct attempts to unify / compare
io.states and stores at compile time.  (That said, programs that attempt to do
that are already pretty contrived ...)

The diff looks fine.

Julien.