std.data.json formal review

[Sönke Ludwig via Digitalmars-d]

Am 18.08.2015 um 00:21 schrieb Andrei Alexandrescu:
> I'll preface my review with a general comment. This API comes at an
> interesting juncture; we're striving as much as possible for interfaces
> that abstract away lifetime management, so they can be used comfortably
> with GC, or at high performance (and hopefully no or only marginal loss
> of comfort) with client-chosen lifetime management policies.
>
> The JSON API is a great test bed for our emerging recommended "push
> lifetime up" idioms; it's not too complicated yet it's not trivial
> either, and has great usefulness.
>
> With this, here are some points:
>
> * All new stuff should go in std.experimental. I assume "stdx" would
> change to that, should this work be merged.

Check.

>
> * On the face of it, dedicating 6 modules to such a small specification
> as JSON seems excessive. I'm thinking one module here. (As a simple
> point: who would ever want to import only foundation, which in turn has
> one exception type and one location type in it?) I think it shouldn't be
> up for debate that we must aim for simple and clean APIs.

That would mean a single module that is >5k lines long. Spreading out 
certain things, such as JSONValue into an own module also makes sense to 
avoid unnecessarily large imports where other parts of the functionality 
isn't needed. Maybe we could move some private things to "std.internal" 
or similar and merge some of the modules?

But I also think that grouping symbols by topic is a good thing and 
makes figuring out the API easier. There is also always package.d if you 
really want to import everything.

> * stdx.data.json.generator: I think the API for converting in-memory
> JSON values to strings needs to be redone, as follows:
>
> - JSONValue should offer a byToken range, which offers the contents of
> the value one token at a time. For example, "[ 1, 2, 3 ]" offers the '['
> token followed by three numeric tokens with the respective values
> followed by the ']' token.

An input range style generator is on the TODO list, but would a token 
range be really useful for anything in practice? I would just go 
straight for a char range.

Another thing I'd like to add is an output range that takes parser nodes 
and writes to a string output range. This would be the kind of interface 
that would be most useful for a serialization framework.

> - On top of byToken it's immediate to implement a method (say toJSON or
> toString) that accepts an output range of characters and formatting
> options.
>
> - On top of the method above with output range, implementing a toString
> overload that returns a string for convenience is a two-liner. However,
> it shouldn't return a "string"; Phobos APIs should avoid "hardcoding"
> the string type. Instead, it should return a user-chosen string type
> (including reference counting strings).

Without any existing code to test this against, how would this look 
like? Simply using an `Appender!rcstring`?

>
> - While at it make prettyfication a flag in the options, not its own
> part of the function name.

Already done. Pretty printing is now the default and there is 
GeneratorOptions.compact.

> * stdx.data.json.lexer:
>
> - I assume the idea was to accept ranges of integrals to mean "there's
> some raw input from a file". This seems to be a bit overdone, e.g.
> there's no need to accept signed integers or 64-bit integers. I suggest
> just going with the three character types.

It's funny you say that, because this was your own design proposal.

Regarding the three character types, if we drop everything but those, I 
think we could also go with Walter's suggestion and just drop everything 
apart from "char". Putting a conversion range from dchar to char would 
be trivial and should be fast enough.

> - I see tokenization accepts input ranges. This forces the tokenizer to
> store its own copy of things, which is no doubt the business of
> appenderFactory.  Here the departure of the current approach from what I
> think should become canonical Phobos APIs deepens for multiple reasons.
> First, appenderFactory does allow customization of the append operation
> (nice) but that's not enough to allow the user to customize the lifetime
> of the created strings, which is usually reflected in the string type
> itself. So the lexing method should be parameterized by the string type
> used. (By default string (as is now) should be fine.) Therefore instead
> of customizing the append method just customize the string type used in
> the token.

Okay, sounds reasonable if Appender!rcstring is just going to work.

> - The lexer should internally take optimization opportunities, e.g. if
> the string type is "string" and the lexed type is also "string", great,
> just use slices of the input instead of appending them to the tokens.

It does.

> - As a consequence the JSONToken type also needs to be parameterized by
> the type of its string that holds the payload. I understand this is a
> complication compared to the current approach, but I don't see an out.
> In the grand scheme of things it seems a necessary evil: tokens may or
> may not need a means to manage lifetime of their payload, and that's
> determined by the type of the payload. Hopefully simplifications in
> other areas of the API would offset this.

It wouldn't be too bad here, because it's presumably pretty rare to 
store tokens or parser nodes. Worse is JSONValue.

> - At token level there should be no number parsing. Just store the
> payload with the token and leave it for later. Very often numbers are
> converted without there being a need, and the process is costly. This
> also nicely sidesteps the entire matter of bigints, floating point etc.
> at this level.

Okay, again, this was your own suggestion. The downside of always 
storing the string representation is that it requires allocations if no 
slices are used, and that the string will have to be parsed twice if the 
number is indeed going to be used. This can have a considerable 
performance impact.

> - Also, at token level strings should be stored with escapes unresolved.
> If the user wants a string with the escapes resolved, a lazy range does it.

To make things efficient, it currently stores escaped strings if slices 
of the input are used, but stores unescaped strings if allocations are 
necessary anyway.

> - Validating UTF is tricky; I've seen some discussion in this thread
> about it. On the face of it JSON only accepts valid UTF characters. As
> such, a modularity-based argument is to pipe UTF validation before
> tokenization. (We need a lazy UTF validator and sanitizer stat!) An
> efficiency-based argument is to do validation during tokenization. I'm
> inclining in favor of modularization, which allows us to focus on one
> thing at a time and do it well, instead of duplicationg validation
> everywhere. Note that it's easy to write routines that do JSON
> tokenization and leave UTF validation for later, so there's a lot of
> flexibility in composing validation with JSONization.

It's unfortunate to see this change of mind in face of the work that 
already went into the implementation. I also still think that this is a 
good optimization opportunity that doesn't really affect the 
implementation complexity. Validation isn't duplicated, but reused from 
std.utf.

> - Litmus test: if the input type is a forward range AND if the string
> type chosen for tokens is the same as input type, successful
> tokenization should allocate exactly zero memory. I think this is a
> simple way to make sure that the tokenization API works well.

Supporting arbitrary forward ranges doesn't seem to be enough, it would 
at least have to be combined with something like take(), but then the 
type doesn't equal the string type anymore. I'd suggest to keep it to 
"if is sliceable and input type equals string type", at least for the 
initial version.

> - If noThrow is a runtime option, some functions can't be nothrow (and
> consequently nogc). Not sure how important this is. Probably quite a bit
> because of the current gc implications of exceptions. IMHO: at lexing
> level a sound design might just emit error tokens (with the culprit as
> payload) and never throw. Clients may always throw when they see an
> error token.

noThrow is a compile time option and there are @nothrow unit tests to 
make sure that the API is @nothrow at least for string inputs.

> * stdx.data.json.parser:
>
> - Similar considerations regarding string type used apply here as well:
> everything should be parameterized with it - the use case to keep in
> mind is someone wants everything with refcounted strings.

Okay.

> - The JSON value does its own internal allocation (for e.g. arrays and
> hashtables), which should be fine as long as it's encapsulated and we
> can tweak it later (e.g. make it use reference counting inside).

Since it's based on (Tagged)Algebraic, the internal types are part of 
the interface. Changing them later is bound to break some code. So AFICS 
this would either require to make the types used parameterized (string, 
array and AA types). Or to abstract them away completely, i.e. only 
forward operations but deny direct access to the type.

... thinking about it, TaggedAlgebraic could do that, while Algebraic can't.

> - parseJSONStream should parameterize on string type, not on
> appenderFactory.

Okay.

> - Why both parseJSONStream and parseJSONValue? I'm thinking
> parseJSONValue would be enough because then you trivially parse a stream
> with repeated calls to parseJSONValue.

parseJSONStream is the pull parser (StAX style) interface. It returns 
the contents of a JSON document as individual nodes instead of storing 
them in a DOM. This part is vital for high-performance parsing, 
especially of large documents.

> - FWIW I think the whole thing with accommodating BigInt etc. is an
> exaggeration. Just stick with long and double.

As mentioned earlier somewhere in this thread, there are practical needs 
to at least be able to handle ulong, too. Maybe the solution is indeed 
to just (optionally) store the string representation, so people can 
convert as they see fit.

> - readArray suddenly introduces a distinct kind of interacting -
> callbacks. Why? Should be a lazy range lazy range lazy range. An adapter
> using callbacks is then a two-liner.

It just has a more complicated implementation, but is already on the 
TODO list.

> - Why is readBool even needed? Just readJSONValue and then enforce it as
> a bool. Same reasoning applies to readDouble and readString.

This is for lower level access, using parseJSONValue would certainly be 
possible, but it would have quite some unneeded overhead and would also 
be non- at nogc.

> - readObject is with callbacks again - it would be nice if it were a
> lazy range.

Okay, is also already on the list.

> - skipXxx are nice to have and useful.
>
> * stdx.data.json.value:
>
> - The etymology of "opt" is unclear - no word starting with "opt" or
> obviously abbreviating to it is in the documentation. "opt2" is awkward.
> How about "path" and "dyn", respectively.

The names are just placeholders currently. I think one of the two should 
also be enough. I've just implemented both, so that both can be 
tested/seen in practice. There have also been some more name suggestions 
in a thread mentioned by Meta with a more general suggestion for normal 
D member access. I'll see if I can dig those up, too.

> - I think Algebraic should be used throughout instead of
> TaggedAlgebraic, or motivation be given for the latter.

There have already been quite some arguments that I think are 
compelling, especially with a lack of counter arguments (maybe their 
consequences need to be explained better, though). TaggedAlgebraic could 
also (implicitly) convert to Algebraic. An additional argument is the 
potential possibility of TaggedAlgebraic to abstract away the underlying 
type, since it doesn't rely on a has!T and get!T API.

But apart from that, algebraic is unfortunately currently quite unsuited 
for this kind of abstraction, even if that can be solved in theory (with 
a lot of work). It requires to write things like 
obj.get!(JSONValue[string])["foo"].get!JSONValue instead of just 
obj["foo"], because it simply returns Variant from all of its forwarded 
operators.

> - JSONValue should be more opaque and not expose representation as much
> as it does now. In particular, offering a built-in hashtable is bound to
> be problematic because those are expensive to construct, create garbage,
> and are not customizable. Instead, the necessary lookup and set APIs
> should be provided by JSONValue whilst keeping the implementation
> hidden. The same goes about array - a JSONValue shall not be exposed;
> instead, indexed access primitives should be exposed. Separate types
> might be offered (e.g. JSONArray, JSONDictionary) if deemed necessary.
> The string type should be a type parameter of JSONValue.

This would unfortunately at the same time destroy almost all benefits 
that using (Tagged)Algebraic has, namely that it would opens up the 
possibility to have interoperability between different data formats (for 
example, passing a JSONValue to a BSON generator without letting the 
BSON generator know about JSON). This is unfortunately an area that I've 
also not yet properly explored, but I think it's important as we go 
forward with other data formats.

>
> ==============================
>
> So, here we are. I realize a good chunk of this is surprising ("you mean
> I shouldn't create strings in my APIs?"). My point here is, again, we're
> at a juncture. We're trying to factor garbage (heh) out of API design in
> ways that defer the lifetime management to the user of the API.

Most suggestions so far sound very reasonable, namely parameterizing 
parsing/lexing on the string type and using ranges where possible. 
JSONValue is a different beast that needs some more thought if we really 
want to keep it generic in terms of allocation/lifetime model.

In terms of removing "garbage" from the API, I'm just not 100% sure if 
removing small but frequently used functions, such as a string 
conversion function (one that returns an allocated string) is really a 
good idea (what Walter's suggested).

>
> We could pull json into std.experimental and defer the policy decisions
> for later, but I think it's a great driver for them. (Thanks Sönke for
> doing all the work, this is a great baseline.) I think we should use the
> JSON API as a guinea pig for the new era of D API design in which we
> have a solid set of principles, tools, and guidelines to defer lifetime
> management. Please advise.