hash table handles

hash table handles

[Paul Jarc]

If I have a pair returned by hash*-get-handle or hash*-create-handle,
can I use set-cdr! on it and expect to see the change reflected in
future lookups of the same key?


paul


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Re: hash table handles

[Paul Jarc]

I wrote:
> If I have a pair returned by hash*-get-handle or hash*-create-handle,
> can I use set-cdr! on it and expect to see the change reflected in
> future lookups of the same key?

To clarify: I can see by experimentation that the code does behave
this way.  But the documentation doesn't make it entirely clear
whether this is by design, or an accident, and thus whether it might
change in the future.


paul


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Re: hash table handles

[Thien-Thi Nguyen]

   From: prj@po.cwru.edu (Paul Jarc)
   Date: Mon, 04 Nov 2002 18:56:21 -0500

   But the documentation doesn't make it entirely clear whether this is
   by design, or an accident, and thus whether it might change in the
   future.

using cdr to access the associated data should be fine.  i see this:

  By convention, a handle in a hash table is the pair which associates a
  key with a value.

how would you change this to be more clear?

thi


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Re: hash table handles

[Paul Jarc]

Thien-Thi Nguyen <ttn@giblet.glug.org> wrote:
> using cdr to access the associated data should be fine.

I had no doubt of that.  My doubt is about set-cdr!.

> i see this:
>
>   By convention, a handle in a hash table is the pair which associates a
>   key with a value.

I don't see that in the 1.6.0 manual, so I'll assume it was added
later.  Assuming the following is in fact true, I would also like to
see something to the effect of:
  Changes to the cdr of a handle (e.g., via set-cdr!) will be
  reflected in future lookups for the same key.  Changes to the car of
  a handle (e.g., via set-car!) may cause future lookups to fail.


paul


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Re: hash table handles

[Thien-Thi Nguyen]

   From: prj@po.cwru.edu (Paul Jarc)
   Date: Tue, 05 Nov 2002 12:15:51 -0500

   Thien-Thi Nguyen <ttn@giblet.glug.org> wrote:
   > using cdr to access the associated data should be fine.

   I had no doubt of that.  My doubt is about set-cdr!.

sorry, i wasn't clear: "using cdr" should be "using the cdr", i.e., read
via "cdr", write via "set-cdr!".

   I don't see that in the 1.6.0 manual, so I'll assume it was added
   later.

see 1.4.1.91 manual, which derives from 1.5.7 and HEAD, mostly.  the
referenced portion is part of the hash*-get-handle docstrings harvested
automagically from libguile/hashtab.c by "make" in doc/ref/.

     Changes to the cdr of a handle (e.g., via set-cdr!) will be
     reflected in future lookups for the same key.  Changes to the car
     of a handle (e.g., via set-car!) may cause future lookups to fail.

or, "behavior after changing the car of a handle is unspecified."

thi


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