Although in this case - I'm digging into the 'standard' interface provided
by DataType, but I agree this does not seem correct. I'm going to submit a
pull request so that the methods for assoitaive for keytype and valtype are
exported which resolves this specific issue. The type stable generated
function may not be a bad idea to have in the toolkit.
On Thursday, September 3, 2015 at 10:36:27 AM UTC-4, Jarrett Revels wrote:
>
> Though how would one correctly access the 'nth' elements type ?
>> It brings up a question regarding parameterized types, how should (or
>> should) one access/refer to the parameters of the type rather than an
>> instance. It seems like a bad (queezy) practice to dig into the parameters
>> of a type?
>
>
> It's generally bad practice to access fields of objects that aren't
> "yours." In the case of the keytype function you defined, for example, it
> causes type instability. The more Julian (and type stable way) to define
> such a function is:
>
> keytype{K,V}(::Type{Dict{K,V}}) = K
> keytype(dict::Dict) = keytype(typeof(dict))
>
> If you *really* need a general, type stable way to access the nth
> parameter of a type, your best bet will probably be doing something like
> this:
>
> @generated function getparam{T,n}(::Type{T}, ::Type{Val{n}})
> P = T.parameters[n]
> return :($P)
> end
>
> Though be warned: naive callers of this method might call it in a type
> unstable manner anyway by passing the index "n" from the value domain
> rather than the type domain.
>
> Best,
> Jarrett
>
> On Thursday, September 3, 2015 at 10:04:22 AM UTC-4, Michael Francis wrote:
>>
>> I will likely do so, it's quite a common action to take.
>>
>> Fortunately with typeof( ( 1,2 ) ) == Tuple{Int64,Int64} there is a
>> significantly greater consistency in 0.4 vs 0.3
>>
>> Though how would one correctly access the 'nth' elements type ?
>>
>> It brings up a question regarding parameterized types, how should (or
>> should) one access/refer to the parameters of the type rather than an
>> instance. It seems like a bad (queezy) practice to dig into the parameters
>> of a type?
>>
>> On Thursday, September 3, 2015 at 9:41:18 AM UTC-4, Tim Holy wrote:
>>>
>>> base/dict.jl defines keytype and valtype, but they are not exported. You
>>> could
>>> file a pull request that exports them (it would be a 2-line patch,
>>> though you
>>> might want to add a test to make sure they stay exported).
>>>
>>> --Tim
>>>
>>> On Thursday, September 03, 2015 06:37:02 AM Michael Francis wrote:
>>> > In the short term I have defined the following in the offending
>>> package for
>>> > v0.4 only
>>> >
>>> > function keytype( dict )
>>> > return eltype( dict ).parameters[1]
>>> > end
>>> >
>>> > I agree that a standard protocol of accessing the key and value types
>>> of a
>>> > pair / associative is the way to go.
>>> >
>>> > On Thursday, September 3, 2015 at 9:31:39 AM UTC-4, Matt Bauman wrote:
>>> > > Oh man that's tricky. The trouble is that you're effectively saying
>>> > > `Pair{Symbol,Int}[1]`, which is the syntax for a typed array:
>>> > > Pair{Symbol,Int}[:x=>1, :y=>2]. One way around this is to define:
>>> > >
>>> > > keytype{A,B}(::Type{Pair{A,B}}) = A
>>> > > valuetype{A,B}(::Type{Pair{A,B}}) = B
>>> > > pairtypes{A,B}(::Type{Pair{A,B}}) = (A,B)
>>> > >
>>> > > If you need this to work on 0.3, too, you can easily make these
>>> functions
>>> > > work for the old-style Tuples, too.
>>> > >
>>> > > On Thursday, September 3, 2015 at 9:06:30 AM UTC-4, Michael Francis
>>> wrote:
>>> > >> Incidentally
>>> > >>
>>> > >> eltype( Pair{String,Float64} )
>>> > >>
>>> > >> gives Any, that seems slightly strange as well .
>>> > >>
>>> > >> On Thursday, September 3, 2015 at 9:02:33 AM UTC-4, Michael Francis
>>> wrote:
>>> > >>> julia> eltype( Dict( :x => 1, :y => 2 ) )[1]
>>> > >>> ERROR: MethodError: `convert` has no method matching
>>> convert(::Type{Pair
>>> > >>> {Symbol,Int64}}, ::Int64)
>>> > >>> This may have arisen from a call to the constructor
>>> Pair{Symbol,Int64
>>> > >>> }(...),
>>> > >>> since type constructors fall back to convert methods.
>>> > >>>
>>> > >>> Closest candidates are:
>>> > >>> Pair{A,B}(::Any, ::Any)
>>> > >>> call{T}(::Type{T}, ::Any)
>>> > >>> convert{T}(::Type{T}, ::T)
>>> > >>>
>>> > >>> in getindex at array.jl:167
>>> > >>>
>>> > >>> Is this intentional ? This breaks a package I am dependent on - I
>>> > >>> believe the assumption was that Pair would respect the tuple API,
>>> this
>>> > >>> appears to not be the case ?
>>> > >>>
>>> > >>> collect( eltype( Dict( :x => 1, :y => 2 ) ) )
>>> > >>> ERROR: MethodError: `start` has no method matching
>>> start(::Type{Pair{
>>> > >>> Symbol,Int64}})
>>> > >>>
>>> > >>> in collect at array.jl:255
>>> > >>> in collect at array.jl:262
>>>
>>>
