Le lundi 05 janvier 2015 à 18:53 -0800, Greg Plowman a écrit :
>
> The only reason I can think of is that a copy may be
> costly for certain
> types, and it's not needed in most cases since the
> summation will create
> a new value in the general case. But as you noted this
> is not true when
> the array only contains one element. So it looks like
> the most efficient
> fix would be to copy only when n == 1 in
> _mapreduce().
>
>
>
> I must admit I don't really understand the code, however it doesn't
> look like evaluation would be affected for n>=2.
> The extra cost would only be for 1-element arrays:
>
>
> Apparently, for 1-element arrays, zero(::MyType) needs to be defined
> For 0-element arrays, both zero(::MyType) and zero(::Type{MyType})
> need to be defined
> (strangely, for 0-element arrays, mr_empty() calls r_promote(::AddFun,
> zero(T)) which effectively calls zero(T) + zero(zero(T)), so both
> forms of zero() need to be defined
Yes, but that's not an issue as the definitions are equivalent:
zero(x::Number) = oftype(x,0)
zero{T<:Number}(::Type{T}) = convert(T,0)
help?> oftype
Base.oftype(x, y)
Convert "y" to the type of "x" ("convert(typeof(x), y)").
> In any case, at the moment I guess I have 2 workarounds:
>
>
> I could define MyType as a subtype of Number and provide zero()
> functions.
> However, I'm not sure what the side effects of subtyping are, and
> whether this is advisable?
I don't think it would be a problem, it may well make a lot of sense if
your type is similar to a number (which is apparently the case since you
can sum it).
>
> type MyType <:Number
> x::Int
> end
>
> Base.zero(::Type{MyType}) = MyType(0) # required for sum(0-element
> array)
> Base.zero(::MyType) = MyType(0) # required for sum(0-element
> array) and sum(1-element array)
>
> +(a::MyType, b::MyType) = MyType(a.x + b.x)
>
>
>
>
> Alternatively, I could define my own sum() functions, but then if I
> want the general functionality of all variants of sum(), this seems
> non-trivial.
Indeed. Another solution is to make a pull request with a possible fix,
it could be included quite soon in a 0.3.x minor release so that you can
use it.
Regards
