This page written by [DKF]
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What is good about [Tcl]'s arrays?  Their compact syntax for common operations.

What is not so good about [Tcl]'s arrays?  You have to use a hash-table for the mapping from keys to values.

Would would make things extra cool?  If we could take arrays and put a new back-end on them based on [list]s, [dict]ionaries, or even something more exotic.  There have been attempts to do this in the past ([BLT]'s vectors spring to mind here)

Why do this to arrays instead of putting magic conversion on values?  We'd have a proper named location to store the metadata describing how the array is implemented.

What is going to be hard?  Well, [trace]s are definitely tricky, as is [upvar] and friends, as they both (currently) require that your mapping mechanism hands out references to a '''Var''' structure, and that's not something that you can really map nicely onto a [Tcl_Obj] as it is an updatable structure.
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'''What Operations are Needed?'''

   get:   Retrieve a value from the mapping given a key.  This operation should succeed if the key was one returned by the '''list keys''' operation below; behaviour for other keys is undefined in general.
   set:   Update or insert a value into the mapping given a key
   unset:   Update the mapping so as to remove an element
   list keys:   Get the list of keys that map to data values.  Note that an array might be permitted to support other keys with "magic" names, but this operation should only list the keys that map in a straight-forward fashion.
   serialize:   Convert entire array to a string (preferably in the order listed by the '''list keys''' operation.)
   deserialize:   Convert string to array

[[...]]

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[RS] Don't [array]s (and [dict]s) represent a string -> value mapping, while vectors (and [list]s) do (0<=int<maxint) -> value?

[DKF]: So what if vectors/lists have a restricted language of keys?  :^)