Re: [Scheme-reports] Legacy caar to cddddr

[Ray Dillinger <bear@x>]

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On 10/24/2011 07:08 AM, Alex Shinn wrote:

> I'd be interested to see real-world examples of (set-car! (c[ad]{3,}r x) y).
> My guess is these are examples of abusing lists as records.


I am not convinced that using lists (more generally, trees) as
records is in fact an abuse.

With trees you can easily do some kinds of things that simple
records cannot as easily do.  Serialization has been mentioned,
but application, execution, extension, collation, flattening,
etc, are also in this category. There are some types of aggregate
data where native support for those operations is useful, or at
least convenient, and therefore a decision to represent those aggregates
as trees is not an abuse.

Prominent amongst the kinds of data where trees are the most
useful and reasonable expression is program source code, which
is easily and simply handled in macros by these operations.

Alternatives such as destructuring, while syntactically more
elegant for manipulating source code, cannot replace these
procedures completely because they are not applicable in other
contexts and have not so far shown extensions or implementation
that gives them equivalent flexibility, particularly with
respect to mutation.

Alternative naming schemes, such as "first, second, third ....",
while perhaps simpler to explain to a new learner, cannot
fully replace these procedures because they are strictly
list accessors and useless for working with more general
trees.

It's my opinion that keeping all of the procedures c(a|d){1,4}r
is not harmful and not a burden on implementors, and that doing
otherwise constitutes a completely unwarranted and undesirable
change in the standard.


Bear


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