[This (part A) is probably not implemented yet, and probably not even a
new idea.. possibly julia-dev material?]
On mið 23.sep 2015 16:26, Milan Bouchet-Valat wrote:
Le mercredi 23 septembre 2015 à 07:38 -0700, Páll Haraldsson a écrit :
instead of these two type-unstable variants:
type MyType
s::String
end
or of
type MyType
s::Union(UTF8String,UTF16String)
end
A.
Are these (almost*) equivalent? For speed at least (generate same
machine code)? [To each other, but not to having only one type - type
stable.]
That may have been a misunderstanding of mine, because it doesn't have
to be that way.
I know String (abstract) can have any number of concrete implementation,
but at any point in time, just a few, and I know more than two (e.g.
UTF32String).
With a Union or two (or any limited number of types) it seems what extra
indirection (converting to if or "switch", and possibly borrowing bits
in the pointers), might be avoided, by clever optimization, that I guess
are not there - yet..
But in the former case and say there were only these two subtypes, it is
not safe to do these optimizations? Because you can always add more
subtypes at runtime.. Possibly until you actually do that, you could,
but still then you would have to recompile code at that point..
B.
* I see the former is nicer, I was going to test with @code_native
myself (but only on 0.3), but with the latter type:
b=MyType2("Palli")
ERROR: `convert` has no method matching
convert(::Type{Union(UTF16String,UTF8String)}, ::ASCIIString)
in MyType2 at no file
julia> b=MyType2(UTF8String("Palli"))
ERROR: `convert` has no method matching convert(::Type{Array{Uint8,1}},
::ASCIIString)
you may have intended to import Base.convert
in UTF8String at no file
I guess you just need a constructor. I also wander if that could be
automatically generated for you..
--
Palli.
