Did Julia community do something to improve its correctness?

Raf · August 5, 2023, 9:26pm

If you look at the example in the issue, the mutable functor in question is user-defined:

github.com/JuliaLang/julia

`reverse(Iterators.filter)` can be incorrect if predicate is impure

opened 01:17PM - 06 Jul 23 UTC

closed 07:09PM - 07 Aug 23 UTC

adienes

iteration

From docstring of `reverse` > `reverse(itr)` is an iterator over the same col…lection but in the reverse order. yet consider ``` julia> mutable struct RollingMax m end julia> function (rm::RollingMax)(x) if x > rm.m rm.m = x return true else return false end end julia> arr = rand(10); julia> Iterators.filter(RollingMax(0), arr) |> collect 5-element Vector{Float64}: 0.20543016348395104 0.40777744342276 0.6118207748963927 0.6765659450947831 0.8719846507332757 julia> Iterators.filter(RollingMax(0), arr) |> Iterators.reverse |> collect 2-element Vector{Float64}: 0.839252025474182 0.8719846507332757 ``` personally I think `reverse(f::Filter)` should just be removed; it cannot be implemented correctly without knowing the purity of the predicate. another option is to add a warning in the docs? or maybe a third option is to somehow dispatch `reverse` only when `flt` can be proven pure---not sure how this is technically possible though.

And passed into Iterators.filter and Iterators.reverse.

As I’ve mentioned above statements like “seems like a totally unholdable claim” don’t help this conversation… we do actually want to know in what language you can both

do that easily
and
it correctly handles the distinction between a pure function and an impure function

Then we have a clear comparison to make, and maybe something to learn from