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Re: guile-json 2.0.0 released

From: Thompson, David
Subject: Re: guile-json 2.0.0 released
Date: Thu, 13 Dec 2018 11:56:07 -0500

On Thu, Dec 13, 2018 at 11:35 AM John Cowan <address@hidden> wrote:
> On Thu, Dec 13, 2018 at 9:31 AM Thompson, David <address@hidden> wrote:
>> * They have no read syntax
>> * They use a procedural, mutable API
>> * They are slower than alists when the number of keys is small, which
>> is 99% of the time when dealing with serializing objects
> I agree with these objections.
>> Why not do something like Racket does and use vectors for JSON arrays
>> and alists for JSON objects?
> In fact Racket uses hash tables, but it provides a (non-standard) lexical
> syntax for them: #hasheq followed by an a-list.

Ah, okay. I missed this detail in the docs. (I've never actually used Racket)

>> It's not the ideal API IMO, but this way
>> only core data types with read syntax are used and is an improvement
>> over using hash tables.
> My objection to using vectors for JSON arrays is that they are often built up
> element by element, which JavaScript arrays support but Scheme vectors
> do not.  So in practice you would create a list and then convert it to a 
> vector.

Yes, I agree.  For my own uses, I have avoided vectors for this
reason, and also the reason that typically, when serializing, the user
has a list of something, not a vector of something.

> While the same thing is sometimes done with objects, it is more common
> for an object to have a reasonably fixed structure, which can be well
> represented as a vector of pairs, the vector equivalent of an alist.
> When the elements of the object are variable, they can be built up
> as an alist and then converted with list->vector.
> So given that we are changing the structure anyhow, I recommend this
> approach.

I think I understand what you are saying, but to me it has similar
shortcomings as other options due the need of a something->vector

An approach I've used in the past is to tag alists with a special
symbol so that they can be distinguished from regular lists.
Something like this:

'(@ ("foo" . 1) ("bar" . 2))

The assoc-ref procedure will happily work with this structure, so
reading parsed data requires nothing special.  And for serialization
procedures it is simple to add the '@' tag.  I'm almost always using
quasiquote in my serializers:

`(@ ("foo" . ,(thing-foo thing)) ("bar" . ,(thing-bar thing)))

There's an old patch of mine to add JSON to the Guile standard library
that uses this approach.  Unfortunately I was never able to get the
code to meet the standards for merging.

- Dave

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