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Co-authored-by: yoff <lerchedahl@gmail.com>
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Rasmus Wriedt Larsen
2022-09-15 15:13:08 +02:00
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docs/ql-design-patterns.md
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@@ -59,8 +59,8 @@ Now, a concrete subclass can derive from `MySpecialExpr::Range` if it wants to e
Let's use an example from the Python libraries: https://github.com/github/codeql/blob/46751e515c40c6b4c9b61758cc840eec1894a624/python/ql/lib/semmle/python/Concepts.qll#L601-L683
`Escaping`, as the name suggests, models various APIs that escape meta-characters. It has a member-predicate `getKind()` that tells you what sort of escaping the modeled function does. For example, if the result of that predicate is `"html"`, then this means that the escaping function is meant to make things safe to embed inside HTML.
`Escaping::Range` is subclassed to model various APIs, and `kind()` is implemented accordingly.
But we can also subclass `Escaping` to, as in the above example, talk about all HTML-escaping functions.
`Escaping::Range` is subclassed to model various APIs, and `kind()` is implemented accordingly (this typically happens in library models).
But we can also subclass `Escaping`, as in the above example, where `HtmlEscaping` represents all HTML-escaping functions.
You can, of course, do the same without the `::Range` pattern, but it's a little cumbersome:
If you only had an `abstract class Escaping { ... }`, then `HtmlEscaping` would need to be implemented in a slightly tricky way to prevent it from extending `Escaping` (instead of refining it). You would have to give it a charpred `this instanceof Escaping`, which looks useless but isn't. And additionally, you'd have to provide trivial `none()` overrides of all the abstract predicates defined in `Escaping`. This is all pretty awkward, and we can avoid it by distinguishing between `Escaping` and `Escaping::Range`.