NEWS.md

Feature updates for Traits.jl

19 May 2015

Updated to post-tupocalypse Julia (#10380)

17 April 2015

The merge of PR #10380 will necessitate quite some work. Until things have settled a bit, Traits is staying on Julia commit 58cef56f523.

14 April 2015

Major overhaul of istrait function. Now it uses custom-programmed checks instead of method_exists. This allows parameterized methods to be used both in the @traitdef and in the trait implementation. This may have pushed Traits.jl into usable terrain. Closes issues #2 and #8.

30 March 2015

• Allowing symbols and isbits as associated types.

14 Nov 2014

• added ability to make type constructors part of a trait specification. See test at bottom of test/traitdef.jl.

12 Nov 2014

• added associated type, see Haskell or Rust. Examples are in src/commontraits.jl.

• using @doc system for documentation

• improved trait dispatch when ambiguous with the following algorithm:

  • first dispatch on the normal types

  Then dispatch on traits using the following rules, terminating when only one or zero possibilities are left:

  • find all matching traits
  • discriminate using subtraits, i.e. a subtrait will win over its supertrait
  • score all traits according to:
    1 point for all single parameter traits,
    2 points for all two parameter traits,
    etc.
    Now pick the highest scoring method.
  • if still ambiguous throw an error

  See also examples/ex_dispatch.jl.

• better testing of the existence of interface methods. Now uses the method_exists function. This leads to some problems with parameterised types, however it should work fine when the parameters are specified:

  julia> istrait(Indexable{Array})
  false
  
  julia> istrait(Indexable{Array{Int}})
  false
  
  julia> istrait(Indexable{Array{Int,1}})
  true
  

  However there is Julia bug #8959 which can bite here.

  This change also necessitated to introduce a label All (just a type) to be used in interface functions declarations where Any could be used before, for example foo(X, All) -> All. The reason being that signature type checks with method_exists are contravariant, but for Base.return_types covariant (or is it the other way around? Always confusing me...). Example, specify that getindex is defined but we don't care about the type of the second argument. To get this past method_exists:

  julia> method_exists(getindex, (Array{Int,1}, Any))
  false
  
  julia> method_exists(getindex, (Array{Int,1}, None))
  true
  

  So, in the interface specification, we need to write getindex(X, None). Conversely for return types:

  julia> Base.return_types(getindex, (Array{Int,1}, None))
  0-element Array{Any,1}
  
  julia> Base.return_types(getindex, (Array{Int,1}, Any))
  11-element Array{Any,1}:
  Any
  ...
  Any
  

  So now, in the interface specification, we need to write getindex(X, Any). Thus I introduced All which gets replaced by Any or None depending on context.