Have fun: std::function without null#15360
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Like `ref<T>` guarantees a non-null pointer, `fun<Sig>` guarantees a non-null callable. When a callable is nullable, use `std::function` directly — not `optional<fun<>>`. The two types serve complementary roles: `fun<Sig>` for non-null, `std::function<Sig>` for nullable. Construction from callables (lambdas, function pointers) is implicit since these are inherently non-null. Construction from `std::function` is explicit, since it may be empty. `fun(nullptr)` is a deleted overload. A moved-from `fun<>` violates its non-null invariant, matching the precedent set by `ref<T>`. Deleting moves was considered but deemed unnecessary: the goal of this type is to document and enforce an invariant, not to force non-null at all costs including performance. `get_fn()` exposes the underlying `std::function` for API that only exists there, such as `target<>()`.
- `fun` was used for variable/field names in a number of places. These have been replaced by similar or more descriptive names, to avoid name shadowing, which is not allowed in the GCC-based build. - Use in-place construction instead of assignment. - `FilteringSourceAccessor::checkAccess` had a fallback for when `makeNotAllowedError` was null. Since `MakeNotAllowedError` is now `fun<>`, we've proven the null branch is dead code and have removed it. - `src/nix/flake.cc`, `src/nix/search.cc`, `src/nix/ls.cc` are left as-is — the self-referential lambdas there cause too much reindentation for too little benefit.
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A bit churny but oh well.
Ericson2314
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Have `fun`: `std::function` without null
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Motivation
Tracking which functions may or may not be null is not my idea of fun.
This applies the same pattern we have with
ref.Side-catch:
1 line of dead code removed.
Occasional line reductions thanks to the shorter name, and a couple direct initializations. net: -9
Context
I've kept the puns to a minimum.
See commit messages for details.
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