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JuliaFolds / Transducers.jl

Efficient transducers for Julia
https://juliafolds.github.io/Transducers.jl/dev/
MIT License
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`ERROR: Stateful transducer Enumerate(1, 1) does not support "combine"` #561

Open ParadaCarleton opened 1 year ago

ParadaCarleton commented 1 year ago

A bit of a weird one:

julia> x = 1:100
1:100

julia> x |> foldxt(+)
5050

julia> x |> Enumerate() |> Map(last) |> foldxt(+)
ERROR: Stateful transducer Enumerate(1, 1) does not support `combine`
Stacktrace:
  [1] error(::String, ::Enumerate{Int64}, ::String)
    @ Base ./error.jl:44
  [2] combine(rf::Transducers.Reduction{…}, a::Transducers.PrivateState{…}, b::Transducers.PrivateState{…})
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/core.jl:580
  [3] _reduce(ctx::Transducers.NoopDACContext, rf::Transducers.Reduction{…}, init::Transducers.InitOf{…}, reducible::Transducers.SizedReducible{…})
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:160
  [4] _reduce(ctx::Transducers.NoopDACContext, rf::Transducers.Reduction{…}, init::Transducers.InitOf{…}, reducible::Transducers.SizedReducible{…}) (repeats 3 times)
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:155
  [5] _transduce_assoc_nocomplete
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:138 [inlined]
  [6] transduce_assoc(xform::Transducers.Composition{…}, step::Completing{…}, init::Transducers.InitOf{…}, coll0::UnitRange{…}; simd::Val{…}, basesize::Nothing, stoppable::Nothing, nestlevel::Nothing)
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:108
  [7] transduce_assoc
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:84 [inlined]
  [8] foldxt
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:242 [inlined]
  [9] foldxt
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:245 [inlined]
 [10] #188
    @ Transducers ~/.julia/packages/Transducers/xbs8O/src/reduce.jl:248 [inlined]
 [11] |>(x::Transducers.Eduction{Transducers.Reduction{…}, UnitRange{…}}, f::Transducers.var"#188#189"{@Kwargs{}, typeof(+)})
    @ Base ./operators.jl:915
 [12] top-level scope
    @ REPL[42]:1

So, first, why is Enumerate considered stateful even when used on an object that should let Enumerate be stateless (like an array)? Is there a simple way to get this behavior? If not, should there be some kind of transducer for accessing both the keys and values of an array/dictionary, like KeyVal, that is stateless?