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lib.fileset: Document decision for strict existence checks

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Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
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Silvan Mosberger 2023-11-20 17:55:53 +01:00
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lib/fileset/README.md
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@ -252,3 +252,16 @@ The `fileFilter` function takes a path, and not a file set, as its second argume
- (+) That can change depending on which files are included, so if it's used for `fileFilter`
it would change the `subpath`/`components` value depending on which files are included.
- (+) If necessary, this restriction can be relaxed later, the opposite wouldn't be possible
### Strict path existence checking
Coercing paths that don't exist to file sets always gives an error.
- (-) Sometimes you want to remove a file that may not always exist using `difference ./. ./does-not-exist`,
but this does not work because coercion of `./does-not-exist` fails,
even though its existence would have no influence on the result.
- (+) This is dangerous, because you wouldn't be protected against typos anymore.
E.g. when trying to prevent `./secret` from being imported, a typo like `difference ./. ./sercet` would import it regardless.
- (+) `difference ./. (maybeMissing ./does-not-exist)` can be used to do this more explicitly.
- (+) `difference ./. (difference ./foo ./foo/bar)` should report an error when `./foo/bar` does not exist ("double negation"). Unfortunately, the current internal representation does not lend itself to a behavior where both `difference x ./does-not-exists` and double negation are handled and checked correctly.
This could be fixed, but would require significant changes to the internal representation that are not worth the effort and the risk of introducing implicit behavior.