Re: [Sbcl-help] string type uncertainty?

[Pascal B. <pj...@in...>]

Christophe Rhodes writes:
> > I   understand  that  UPGRADED-ARRAY-ELEMENT-TYPE   is  implementatio=
n
> > dependant, but why does sbcl upgrade  a (ARRAY BASE-CHAR) to T and no=
t
> > to (ARRAY BASE-CHAR) ?
>=20
> Because the implementation does not provide a representation of arrays
> specialized to holding arrays of BASE-CHAR.

Further thinking  on this  point, I believe  that there's  a confusion
between the  representation (and UPGRADED-ARRAY-ELEMENT-TYPE  seems to
speak about  that), and the semantic.  Type  inferences and assertions
should be about  semantics.  That is: of course,  I understand that an
array containing arrays would be  implemented as an array of pointers,
so  it  could  as well  have  elements  of  any  kind.  But  when  the
programmer assert that this array is (array (array character *) *), he
says that he's  only considering arrays of arrays  of characters, even
if that kind of array  could, with the same implementation, hold other
things.


> As an example: consider the type (SINGLE-FLOAT 0.0 1.0).  If you ask
> sbcl to create an array :ELEMENT-TYPE '(SINGLE-FLOAT 0.0 1.0), it will
> give you back an array specialized to hold general SINGLE-FLOATs, the
> layout of which looks something like this in memory:
>=20
>   [ header | length | element0 | element1 | element2 ]
>=20
> where the element<n>s are unboxed single floats (i.e. they don't have
> any header; they are just the bare IEEE single float in bits).  This
> is what is meant by a specialized array on some type.

Yes, but  the type inference  engine should remember that  elements of
this  array are  always  between 0.0  and  1.0, and  could apply  this
knowledge to (AREF (THE (ARRAY (SINGLE-FLOAT 0.0 1.0) PROBS) 0).


> You might wish to consider what you would need to do to provide a
> specialization of array to hold objects of type (ARRAY BASE-CHAR).

I  think that  since objects  of type  (ARRAY BASE-CHAR  *) may  be of
different sizes,  an array  of such  objects needs to  be an  array of
pointers to  them.  An  (ARRAY (ARRAY BASE-CHAR  10) *) could  put the
BASE-CHAR inline:

   [ header | length |t|e|n|c|h|a|r|a|c|t|t|e|n|c|h|a|r|a|c|t|...]


The point  is that  while a (ARRAY  (ARRAY BASE-CHAR  *) *) has  to be
implemented as  an (ARRAY T  *), it must  not forget that  it contains
actually (ARRAY BASE-CHAR *).


> > Is there a declaration I could add to specify that all the elements o=
f
> > my array are of type (ARRAY BASE-CHAR) ?
>=20
> No.
>=20
> > Would adding THE (ARRAY CHARACTER *) in the right places do?
>=20
> Yes.

--=20
__Pascal_Bourguignon__                   http://www.informatimago.com/
----------------------------------------------------------------------
Do not adjust your mind, there is a fault in reality.