[plt-scheme] PLT4 expectations

If I understand this well, it is still worthwhile to use pair? in stead of 
list? in case you are sure that the object is either a list or not a pair at 
all.
For a moment I thought I could ,with the same costs, use list? in stead of 
pair? where I in fact I do mean list?
Jos

----- Original Message ----- 
From: "Matthew Flatt" <mflatt at cs.utah.edu>
To: "Doug Orleans" <dougorleans at gmail.com>
Cc: "PLT-Scheme Mailing List" <plt-scheme at list.cs.brown.edu>
Sent: Tuesday, March 11, 2008 1:14 PM
Subject: Re: [plt-scheme] PLT4 expectations


> At Tue, 11 Mar 2008 01:01:08 -0400, Doug Orleans wrote:
>> Matthew Flatt writes:
>>  > At Mon, 10 Mar 2008 14:34:49 +0000, "Paulo J. Matos" wrote:
>>  > > The idea of PLT4 creating by default immutable pairs and the so, 
>> means
>>  > > that it also opens the possibility for far more and better
>>  > > optimizations in the compiler, right?
>>  >
>>  > I doubt that there are many new optimization opportunities that matter
>>  > in practice.
>>
>> Well, you could make "list?" be O(1) instead of O(n), at a cost of one
>> bit per cons cell.
>
> Yes, `list?' is currently amortized constant time.
>
> To avoid making `cons' more expensive, no bit is actually set until you
> start using `list?' (which is why the cost has to be amortized to call
> it constant time).
>
> Matthew
>
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