Many methods in Julia base, especially those that interface with external C libraries like BLAS, LAPACK and FFTW are only defined for StridedArray, which currently is a Union type. It has been argued before by @timholy that properties like strided and contiguous should actually be a trait (a THT).

However, my main question is about the interface for a StridedArray, which is currently incomplete and somewhat inconsistent. The manual describes StridedArray as being an array A such that, increasing index k by 1 is equivalent to linear indexing where the linear index is increased by stride(A,k). But that is of course not true, since linear indexing is expected to be such that the linear index increases by prod(size(A)[1:k-1]). A simple example is A=rand(5,5);B=sub(A,1:3,1:3). Clearly, stride(B,2)=5, but linear indexing with i+5*(j-1) would not give me element (i,j), you need to index with i+3*(j-1) for that. In that respect, stride(A,k) is also defined for a general AbstractArray and just returns prod(size(A)[1:k-1]), which does not correspond to its description in the manual as "distance in memory".

So what is the role of StridedArray in Julia. Is it only to interface with C libraries and to be used in combination with pointer? It is certainly useful to also exploit the stridedness in pure Julia methods, see e.g. permutedims!. Currently, this is dealt with by checking explicitly whether the StridedArray is a SubArray, and in that case, to extract the underlying parent array and the offset, and then apply linear indexing to that. I noticed that parent is now part of the exported method list, but there is no corresponding method to get the offset or first_index. Maybe it would be good to have this method as well and then define these methods (stride and strides, parent, offset?) as the interface for a strided array (as type or as trait).