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Principle: Write only one algorithm to accomplish a task. #562

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88a3e66
Principle: Write only one algorithm to accomplish a task.
jyasskin Mar 7, 2025
6474656
Goal instead of task.
jyasskin Mar 12, 2025
d414b30
Take Martin's suggestions.
jyasskin Mar 12, 2025
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Expand Up @@ -24,6 +24,23 @@ Required IDs: using-http

Link Defaults: html (dfn) queue a task/in parallel/reflect
</pre>
<pre class="biblio">
{
"draft-ietf-oauth-sd-jwt-vc": {
"href": "https://datatracker.ietf.org/doc/html/draft-ietf-oauth-sd-jwt-vc",
"title": "SD-JWT-based Verifiable Credentials (SD-JWT VC)",
"status": "Internet-Draft",
"publisher": "IETF"
},
"vc-data-model-1.1": {
"href": "https://www.w3.org/TR/vc-data-model-1.1/",
"title": "Verifiable Credentials Data Model v1.1",
"status": "REC",
"publisher": "W3C",
"deliveredBy": ["https://www.w3.org/groups/wg/vc"]
}
}
</pre>
<pre class="link-defaults">
spec:css-cascade-5; type:dfn; text:inherit
spec:css2; type: property; text: line-height
Expand Down Expand Up @@ -3488,6 +3505,52 @@ While the best path forward may be to choose not to specify the feature,
there is the risk that some implementations
may ship the feature as a nonstandard API.

<h3 id="multiple-algorithms">Write only one algorithm to accomplish a task</h3>
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Maybe "goal" instead of "task"? This immediately made me think of the event loop.


When specifying how to accomplish a task, write a single algorithm to do it,
instead of letting implementers pick between multiple algorithms.
It is very difficult to ensure that
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It is very difficult to ensure that
It is very difficult to ensure that

two different algorithms produce the same results in all cases,
and doing so is rarely worth the cost.

Multiple algorithms seem particularly tempting when defining
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I don’t think that you need this paragraphs as long as an example mentions a file format.

how to parse file formats,
but they are an equally bad idea in that context.

<div class="example" id="html-polyglot">

[[html-polyglot inline]] was an attempt to define a common subset of both XHTML and HTML
that could be parsed into equivalent DOM trees
using either the [[HTML#the-xhtml-syntax|XHTML parsing]]
or [[HTML#syntax|HTML parsing]] algorithm.
Authors who tried to use this syntax tended to produce documents
that actually only worked with one of the two parsers.
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that actually only worked with one of the two parsers.
that only worked with one of the two parsers.


</div>

<div class="example" id="json-ld-polyglot">

The [[vc-data-model-1.1 inline obsolete]] and [[cid inline]] both endorse implementations
that use either JSON or JSON-LD to parse bytes into their data models.
Because JSON-LD provides many more ways
to assign properties to particular objects than JSON does,
these specifications had to add extra rules to both kinds of parsers
in order to ensure that each input document had exactly one possible interpretation.
[[vc-data-model-2.0 inline]] and [[draft-ietf-oauth-sd-jwt-vc inline]] fixed the problem
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Manu is right that these are completely different (and that they likely represent standardization failure, though the question of where the failure occurred might be contested). In a sense, it is OK that they are completely different (that they are in competition is potentially bad if they address the same use cases, but there is no risk that one might be mistaken for the other).

I think that it would serve this example better to focus only on the CID case.

by defining different media types for JSON-LD vs JSON,
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@BigBlueHat BigBlueHat Mar 11, 2025 •
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This statement is incorrect. The VCDMv2 only has a single media type: application/vc.

Likewise, SD-JWT-VC only has one: application/dc+sd-jwt. However, SD-JWT is not JSON parse-able base format.

Both specifications have parsing algorithms unique to their media types--and specific to their tasks.

It remains unclear how these examples are "polyglot".

which means implementations are instructed to use exactly one of the algorithms.
The authors of [[cid inline]] have decided the extra specification work and testing
are acceptable prices to be able to allow both implementation strategies,
and they have partially mitigated the risks by explicitly stating that
if a single document can conformantly have multiple interpretations,
it's a specification bug.

</div>

Note: While [[rfc6838#section-6|structured suffixes]] define that
a document can be parsed in two different ways,
they do not violate this rule because the results have different data models.
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Isn’t the real difference here that the suffix parsing produces an intermediate result?

i suspect that this is insufficient still, because it doesn’t really get at why suffix parsers exist. That is still somewhat contested, but my view is that intermediate results are rarely able to be processed meaningfully, so they are limited to use in diagnostic tools and the like.


<h3 id="monkey-patching">Avoid monkey patching</h3>
A <dfn export>monkey patch</dfn> layers new functionality on top of an existing specification in a way that extends, overrides, or otherwise modifies the existing specification's behavior.
<a href="https://en.wikipedia.org/wiki/Monkey_patch">Monkey patching</a> <strong>is generally considered bad practice and should be avoided</strong> for the reasons listed below; but is sometimes unavoidable
Expand Down