Can `eltype()` deduce the element type of a generator?

Alexander_Belopolsky · October 13, 2017, 9:46pm

Is that me being dumb or you being terse?

I’ve just looked at how collect(::Generator) is implemented. The first thing it does is inferring the element type:

JuliaLang/julia/blob/84ca8eea42a93aa31f8434d7003058099588bd07/base/array.jl#L652


      
              macro default_eltype(itrt)
                  return :(Any)
              end
          end
          
          _array_for(::Type{T}, itr, ::HasLength) where {T} = Array{T,1}(Int(length(itr)::Integer))
          _array_for(::Type{T}, itr, ::HasShape) where {T} = similar(Array{T}, indices(itr))
          
          function collect(itr::Generator)
              isz = iteratorsize(itr.iter)
              et = @default_eltype(typeof(itr))
              if isa(isz, SizeUnknown)
                  return grow_to!(Array{et,1}(0), itr)
              else
                  st = start(itr)
                  if done(itr,st)
                      return _array_for(et, itr.iter, isz)
                  end
                  v1, st = next(itr, st)
                  collect_to_with_first!(_array_for(typeof(v1), itr.iter, isz), v1, itr, st)
              end

Where the macro @default_eltype expands to a call to Core.Inference.return_type.

So why is collect() allowed to use inference while eltype() is not?

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Can `eltype()` deduce the element type of a generator?

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