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This is an example of "Parse, don't validate" principle [1]. Before, we had a number of `StringSet`s in `DerivationOptions` that were not *actually* allowed to be arbitrary sets of strings. Instead, each set member had to be one of: - a store path - a CA "downstream placeholder" - an output name Only later, in the code that checks outputs, would these strings be further parsed to match these cases. (Actually, only 2 by that point, because the placeholders must be rewritten away by then.) Now, we fully parse everything up front, and have an "honest" data type that reflects these invariants: - store paths are parsed, stored as (opaque) deriving paths - CA "downstream placeholders" are rewritten to the output deriving paths they denote - output names are the only arbitrary strings left Since the first two cases both become deriving paths, that leaves us with a `std::variant<SingleDerivedPath, String>` data type, which we use in our sets instead. Getting rid of placeholders is especially nice because we are replacing them with something much more internally-structured / transparent. [1]: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/ Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo> |
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