Re: [TCLCORE] Unifying list and dict types

[Neil M. <Nei...@no...>]

Hi Andy,

On 1 Feb 2010, at 06:08, Andy Goth wrote:
> (Is this the right forum for this type of discussion?)
> 
> I believe it is possible to merge the list and dict Tcl_Obj types such that a dict is simply a list whose intrep contains some extra indexing information; dict extends list without replacing it, like a subclass.  This change can be made without affecting script-level, API-level, or even binary-level compatibility.  The only observable difference should be a performance improvement in some circumstances from reduced shimmering.

Sounds good. If I understand correctly you add a hashtable to the list rep which maps from string keys to list indices, rather than directly to the associated value? Apologies, I don't have time to read through all the details you posted. This seems like a good approach to me.

> 
> The idea came about while working on processing HTTP headers in Wibble.  Headers can repeat, so I create a key/value list using [lappend] instead of [dict set].  Most of the time header repetition can be neglected, so the resulting list is conveniently accessed using [dict get], which returns the last (usually only) matching entry.  However, this incurs a significant shimmering penalty for every lookup.  If [dict get] were to access a persistent index over the list, rather than insisting on storing the values themselves in the index, this shimmering would not take place.  (The Wibble version I describe hasn't been posted to the Wiki yet.)

Presumably your direct concern could be solved by using [dict lappend] instead of [dict set]? I.e., instead of trying to create a map with duplicate keys, you instead store a list of values for each key.

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