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lib.fileset: Document decision for strict existence checks
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
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@ -252,3 +252,16 @@ The `fileFilter` function takes a path, and not a file set, as its second argume
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- (+) That can change depending on which files are included, so if it's used for `fileFilter`
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it would change the `subpath`/`components` value depending on which files are included.
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- (+) If necessary, this restriction can be relaxed later, the opposite wouldn't be possible
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### Strict path existence checking
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Coercing paths that don't exist to file sets always gives an error.
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- (-) Sometimes you want to remove a file that may not always exist using `difference ./. ./does-not-exist`,
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but this does not work because coercion of `./does-not-exist` fails,
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even though its existence would have no influence on the result.
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- (+) This is dangerous, because you wouldn't be protected against typos anymore.
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E.g. when trying to prevent `./secret` from being imported, a typo like `difference ./. ./sercet` would import it regardless.
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- (+) `difference ./. (maybeMissing ./does-not-exist)` can be used to do this more explicitly.
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- (+) `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.
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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.
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