>
>
> 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
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?
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.
