Matt, Thankyou so much for your great reply. And yes it does make sense now after such a helpful and well-targeted explanation. You explained "colon-lowering" in a "explanation-lowered" way that a non-expert can understand. This is refreshingly thoughtful and helpful. Thanks again. -- Greg
On Tuesday, September 22, 2015 at 11:09:41 PM UTC+10, Matt Bauman wrote: > A colon by itself is simply a synonym for `Colon()`, both on 0.3 and 0.4: > > julia> : > Colon() > > You can use colons in normal function calls and dispatch in both 0.3 and > 0.4: > > julia> f(x::Colon) = x > f(:) > Colon() > > That's what's happening with that sub definition. `sub` acts like > indexing, but it's just a normal function call. And so the Colon() passes > through unchanged in both 0.3 and 0.4. > > Indexing expressions are different. There are some special lowering steps > to handle colon (just in 0.3) and end (in both 0.3 and 0.4). "Lowering" is > an intermediate step between parsing and compilation, and you can see what > happens to an expression during lowering by calling `expand`. The expanded > expressions have some funny things in them (`top(endof)(A)` is a special > internal pseudo-syntax that is kinda-sorta like saying `Base.endof(A)`), > but you can see how the `end` keyword lowers to getindex calls in both 0.3 > and 0.4: > > julia> expand(:(A[end])) > :(getindex(A,(top(endof))(A))) > > julia> expand(:(A[1, end])) > :(getindex(A,1,(top(trailingsize))(A,2))) > > julia> expand(:(A[1, end, 3])) > :(getindex(A,1,(top(size))(A,2),3)) > > So, now to the difference between 0.3 and 0.4: the special lowering step > for colons in indexing expressions was removed. > > # Julia 0.3: > julia> expand(:(A[:])) > :(getindex(A,colon(1,(top(endof))(A)))) > > # Julia 0.4: > julia> expand(:(A[:])) > :(getindex(A,:)) > > You can see that in 0.3, it effectively expanded to `1:end`. But 0.4 just > left the `:` symbol as it was, and it just gets evaluated normally as an > argument to getindex. It's no longer special. So in 0.4, you can > specialize dispatch on Colon indices. > > Make sense? > > On Tuesday, September 22, 2015 at 1:07:01 AM UTC-4, Greg Plowman wrote: >> >> Hi All, >> Thanks all for your replies. >> >> OK I can see this will be much easier in v0.4. >> I will revisit when v0.4 released. >> >> I'm still curious about colon and end >> >>> Colon lowering changed in 0.4, >> >> >> Matt, could you expand on this? How/when is this done in v0.3 vs v0.4? >> >> Does this mean v0.3 code attempting to dispatch on Colon type cannot >> work? (for example the code from subarray.jl quoted below) >> >> sub(A::AbstractArray, I::Union(RangeIndex, Colon)...) = sub(A, ntuple( >> length(I), i-> isa(I[i], Colon) ? (1:size(A,i)) : I[i])...) >> >> I noticed that OffsetArrays (https://github.com/alsam/OffsetArrays.jl >> <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2Falsam%2FOffsetArrays.jl&sa=D&sntz=1&usg=AFQjCNGiyReQIa2MTxw3AB-lqxJiQLAVjA> >> >> jA >> <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2Falsam%2FOffsetArrays.jl&sa=D&sntz=1&usg=AFQjCNGiyReQIa2MTxw3AB-lqxJiQLAVjA>) >> >> defines const (..) = Colon(), presumably to use .. in place of : >> >> Will it work in v0.4? >> How and when is end converted? >> >> Thanks again, >> Greg >> >> >> >> >
