show(::QRCompactWYQ) doesn't work properly

[jiahao]

julia> qrfact(eye(2))[:Q] |> display
2x2 QRCompactWYQ{Float64}:
 1.0  0.0
 0.0  1.0

julia> qrfact(eye(2))[:Q] |> show
[#undef #undef
 #undef #undef]

cc: @andreasnoackjensen - is this because getindex(::QRCompactWYQ, i,j) isn't defined?

[andreasnoack]

At least, it is sufficient to define a getindex method to get show working. Maybe we should define a general getindex{AbstractMatrix{<:Number}} method that works by pre and post multiplying unit vectors.

[jiahao]

I like the idea of a general getindex.

[andreasnoack]

I'll do that. Actually, I have done it, but I need to investigate some ambiguity warnings to figure out which could not show value of type Tuple Julia is thinking of.

[JeffBezanson]

I think a type that can't implement getindex is not really an array. I'm not sure we should suddenly introduce the idea that for all conceivable arrays, multiplying by vectors is more fundamental than getting elements.

[andreasnoack]

even when restricted to AbstractMatrix{<:Number}?

[jiahao]

From the offline discussion, the idea was that QRCompactWYQ isn't actually an AbstractArray since it isn't a container that can implement getindex in less than O(N) time (ref JuliaLang/julia#987).

I guess the real issue is why show() is defined the way it is; maybe it should imitate display() and dump the raw data.

[andreasnoack]

This was the reason to have StoredArray<:AbstractArray. See JuliaLang/julia#6212. If the special storage matrices cannot be allowed to be abstract matrices anymore I think we'd need some kind of AbstractParametrized{T} type to make it possible to define something like the Transpose type in JuliaLang/julia#6837.

[andreasnoack]

Fixed by 8e76a21b5a62266eee445cfe9c9e12b6a0600509