Statistics.middle(Base.Slice(4:6)) fails

[jlapeyre]

julia> Statistics.middle(Base.Slice(4:6))
ERROR: BoundsError: attempt to access 3-element Base.Slice{UnitRange{Int64}} with indices 4:6 at index [1]

This is the method called.

Statistics.jl/src/Statistics.jl

Line 780 in e133202

middle(a::AbstractRange) = middle(a[1], a[end])

It's not clear from AbstractRange whether the first index must be 1:

  AbstractRange{T}


  Supertype for ranges with elements of type T. UnitRange and other types are subtypes of this.

But, Base.Slice violates this assumption. Maybe middle(a::AbstractRange) = middle(first(a), a[end]) is correct.

[nalimilan]

Good catch. Probably better use a[begin] rather than first(x) for consistency and so that an error is thrown for empty ranges.

[jlapeyre]

I found this because of the recent iterations of the interfaces-are-not-defined discussions. I saw a[1], which is suspicious, and asked "what is an AbstractRange ? I still don't know the answer. But, descending through subtypes, I find that one of them violates the assumption on the index of the first element. But, that looks like an ad hoc decision. It doesn't tell me who, strictly speaking, is in violation.