Create @view macro for creating SubArrays via indexing.

[simonbyrne]

Allows writing things like

@sub A[1,2:end]

In order to replicate current indexing behaviour, this actually calls slice.

I'm fairly confident this replicates the end replacing behaviour in

julia/src/julia-syntax.scm

Lines 93 to 123 in c43b5e5

	;; return the appropriate computation for an `end` symbol for indexing
	;; the array `a` in the `n`th index.
	;; `tuples` are a list of the splatted arguments that precede index `n`
	;; `last` = is this last index?
	;; returns a call to endof(a), trailingsize(a,n), or size(a,n)
	(define (end-val a n tuples last)
	(if (null? tuples)
	(if last
	(if (= n 1)
	`(call (top endof) ,a)
	`(call (top trailingsize) ,a ,n))
	`(call (top size) ,a ,n))
	(let ((dimno `(call (top +) ,(- n (length tuples))
	,.(map (lambda (t) `(call (top length) ,t))
	tuples))))
	(if last
	`(call (top trailingsize) ,a ,dimno)
	`(call (top size) ,a ,dimno)))))
	;; replace `end` for the closest ref expression, so doesn't go inside nested refs
	(define (replace-end ex a n tuples last)
	(cond ((eq? ex 'end) (end-val a n tuples last))
	((or (atom? ex) (quoted? ex)) ex)
	((eq? (car ex) 'ref)
	;; inside ref only replace within the first argument
	(list* 'ref (replace-end (cadr ex) a n tuples last)
	(cddr ex)))
	(else
	(cons (car ex)
	(map (lambda (x) (replace-end x a n tuples last))
	(cdr ex))))))

This was discussed in an issue at some point, but I forgot which one.

[tkelman]

should be added to rst docs somewhere if exported

[simonbyrne]

How do I do that? I couldn't figure it out.

[tkelman]

It's somewhat documented in CONTRIBUTING.md I think. The gist of it is pick a place for it in the rst, add just the signature, then make docs should fill in the rest.

[simonbyrne]

Great, thanks.

[simonbyrne]

Done.

[tkelman]

Doing this as a macro means it can't support things like A[1:end-1], can it?

hm what was it about end that needed to be handled by lowering then?

[mbauman]

Nice! Some previous discussion is buried in the Arraypocalype issue and starts here: #13157 (comment)

One question is if the macro should recurse through all non-assignment refs or not. I think I'd prefer the recursive behavior, allowing for @sub begin #= algorithm #= end to easily use views everywhere.

[simonbyrne]

Recursive behaviour is more complicated, and we would need to figure out some possible ambiguities like:

@sub A[B[2:end]]

Should that create subarrays of both A and B?

I figured this was a reasonable start, and recursive behaviour could be added later.

[simonbyrne]

hm what was it about end that needed to be handled by lowering then?

I'm not sure, but this might require input from someone who knows more about the internals.

[tkelman]

I think I was just misreading the caveats about the macro option from the long discussion, it was warned that you'd have to reimplement the end lowering behavior in the macro which is exactly what you've done here. More complicated than just replacing getindex with slice, but not too many lines of code overall. One step closer to having a pure-Julia implementation of all lowering?

[tkelman]

"Creates a SubArray"

[simonbyrne]

good catch.

[cstjean]

#15750 is relevant: use first/last as endpoints for Range indexing

[timholy]

I'm nervous about #15750 because I worry it can't handle all situations. This is certainly safer.

Hopefully in the not too distant future we'll also have to generalize this for #16260, and decide whether to add support for begin.

[timholy]

hm what was it about end that needed to be handled by lowering then?

The point has always been that we couldn't handle sub(A, 2:end) because the lowering looks for the containing Expr(:ref, ...), and when you use end as an argument inside a function there isn't such a containing expression. In this case, the macro gets to work with such an expression, so all is well.

I should also point out that Jeff has made a related proposal, adding A@[2:end] to create a view. On balance, though, I think I prefer @sub, mainly because someday I will probably want to add @index and @stored too (see ArrayIteration.jl).

[simonbyrne]

I should also point out that Jeff has made a related proposal, adding A@[2:end] to create a view. On balance, though, I think I prefer @sub, mainly because someday I will probably want to add @index and @stored too (see ArrayIteration.jl).

The thought just occurred to me that we could use A@sub[2:end], which would simply be syntactic sugar for @sub(A[...])? This would then encompass your other examples.

[tkelman]

There's some sentiment in #13157 that this might be better named @view, ArrayViews use can be qualified in packages.

[simonbyrne]

I originally proposed it as @view, but I thought @sub might be better since, (a) it's 1 character shorter, and (b) it calls sub (okay, it actually calls slice, but I presume we're changing that?).

[simonbyrne]

Also worth providing a link to the original inspiration JuliaArrays/ArrayViews.jl#14 (which I had forgotten).

[simonbyrne]

I've renamed to @view to mirror #16972. I've also changed it so that it expands to true && view(X, inds...), to prevent people redefining view by doing @view(X[inds...]) = ...

[tkelman]

Merge?