Add a ForwardDiff.jl extension - #90
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PatrickHaecker wants to merge 7 commits into
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
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added 3 commits
September 25, 2026 10:46
A direction is an angle modulo a full turn, so the field now counts turns in units of 2^-64 and wraps where the circle does. Every `UInt64` names a direction and every direction has exactly one count, which the old half-turn field did not manage: half turns of 0.5 and 2.5 pointed the same way yet compared unequal and hashed apart. Wrapping also makes the group operation a machine add, and the resolution is uniform instead of thinning out towards a full turn. The count is what the constructor takes. An angle has to be rounded to reach it, so that step is now named at the call site with the `halfturns` keyword, and the old `ComplexInfinity(0.5)` is a `MethodError` rather than a silent reinterpretation. Most code needs neither form, since multiplying by `∞` takes the direction from the other operand: `im*∞` and `(1+im)*∞`. The element type carried no information about the value, only about how the direction had been spelled, so it is gone. It had been standing in for "this infinity lies on the real axis", and that was never what it meant: a zero angle written as a float pointed along the positive real axis just as `ComplexInfinity()` does, yet only the latter could be ordered or divided. The union types that used the parameter as a proxy now leave a `ComplexInfinity` out entirely, so the complex plane carries no order and takes no integer operation, exactly as `Base` treats a `Complex`. The count has 64 bits and an angle in a `Float64` has 53, so the two are kept apart wherever the difference shows. Equality and `hash` read the count, never the angle. `angle` reports on `Base`'s branch of `(-π, π]`. And `show` gives the readable `cispi(h)∞` only where that reads back, and the count itself otherwise, so every printed form evaluates to the value it came from.
Dropping the element type removed the only way to ask a `ComplexInfinity` whether it points along the real axis, and that question was worth keeping: it just belongs to the value rather than to the type. `isreal` takes it over, and `RealInfinity` converts a direction that lies on the axis and throws an `InexactError` otherwise, which is what `Real` does with a `Complex`. So an order or an integer operation is still reachable, by naming the conversion, and an infinity off the axis throws instead of quietly behaving as if the imaginary part were not there.
`Base` returns `NaN` where two reals have no result and `NaN + NaN*im` where either operand is complex, and this package only did half of that. Anything computed *from* a `NotANumber` already came back complex against a complex operand, but every place that *produced* one returned the real value, so `NaN * ComplexInfinity()` and `NotANumber() * ComplexInfinity()` disagreed. `_undefined` picks the value from the two operands and now stands wherever an undefined result is made: a `NaN` argument, a zero times an infinity, two infinities divided, and two infinities added in different directions. Only five of those sites can see a complex operand at all; for the rest the choice is decided at compile time and costs nothing.
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September 25, 2026 14:00
This takes over ideas from JuliaMath#20 created by @cscherrer. So thanks for that contribution! As the PR is 5 years old, quite some things are now obsolete. A nice side-effect was finding and fixing a missing `infpromote` for `Bool`.
This would benefit from someone looking at it who knows ForwardDiff better than I do. Fixes: JuliaMath#13
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This would benefit from someone looking at it who knows ForwardDiff.jl better than I do.
Fixes: #13
Builds on #88.