Yes, you can overload the call operator for arbitrary types. On Mon, Dec 7, 2015 at 6:44 PM, Davide Lasagna <lasagnadav...@gmail.com> wrote:
> Sorry to dig up this old post, but I wonder if this has been > solved/addressed in 0.4? > > Thanks > > > On Friday, June 28, 2013 at 12:51:26 AM UTC+1, Kevin Squire wrote: >> >> Short non-answer: if declare something as a typealias with type >> parameters, you can use it to match types, but (as you've found out) you >> can't use it as a constructor. >> >> I asked a very related question recently, and the response was that using >> them as constructors wouldn't be supported. (I'm still hoping Jeff >> Bezanson reconsiders, but perhaps it's technically challenging or >> inefficient.) >> >> https://github.com/JuliaLang/julia/issues/3427 >> >> If you don't care about matching types, you can just not declare it as a >> typealias, and it will look and act like a constructor. >> >> In the issue above, I got around this limitation by the inelegant >> solution of naming the typealias with an underscore, and creating the >> function as you did above: >> >> typealias _OrderedDict{K,V} OrderedDictBase{K,V,LinkedDictItem, >> LinkedDictItem} >> OrderedDict(K::Type, V::Type) = OrderedDictBase{K,V,LinkedDictItem, >> LinkedDictItem}() >> >> Kevin >> >> On Thursday, June 27, 2013 2:28:00 PM UTC-7, juthoh...@gmail.com wrote: >>> >>> I have a question regarding the way constructors work in parametric >>> typealias. Let me sketch using an example. >>> Suppose I started by defining a general pair type: >>> type Pair{T1,T2} >>> element1::T1 >>> element2::T2 >>> end >>> >>> If no special constructor is defined, as in the case above, Julia >>> provides constructors such that I can create: >>> - pair objects of specified T1 and T2, for example >>> Pair{Int64,Float64}(a,b), provided that a is some Int64 and b is a Float64 >>> - pair objects where T1 and T2 are automatically inferred from the >>> arguments, i.e. Pair(a,b) will result in an object of type >>> Pair{typeof(a),typeof(b)} >>> >>> If I define a special inner constructor to replace the constructor of >>> the first line, I also have to provide an outer constructor that replaces >>> the functionality of the second line. Is my understanding so far correct? >>> >>> Now I add some aliases to the game. If I define >>> typealias IntPair Pair{Int,Int} >>> >>> >>> then Julia automatically provides a constructor such that I can do >>> IntPair(a,b), which will create an object of type Pair{Int,Int} provided >>> that a and b are integers >>> >>> Now I define a parametric typealias, such as >>> typealias EqualPair{T} Pair{T,T} >>> >>> >>> then Julia does provide a constructor if I specify the type, i.e. >>> EqualPair{Int}(a,b) will work if a and b are integers, and returns an >>> object of type Pair{Int,Int}. However, there is no automatic interference >>> of the type T if I do not specify it: >>> EqualPair(a,b) returns >>> "ERROR: type: apply: expected Function, got Type{Pair{T,T}}" >>> >>> And now comes my question: If I now try to define the more general >>> constructor myself, by writing either >>> EqualPair{T}(a::T,b::T)=Pair{T,T}(a,b) >>> or >>> EqualPair{T}(a::T,b::T)=EqualPair{T}(a,b) >>> I get, in both cases, an error: >>> "ERROR: invalid method definition: not a generic function" >>> >>> How should I then define a constructor for EqualPair that is more >>> general than the one provided by Julia? >>> >>
