[ANN] Announcing ForwardMethods.jl

Package Announcements
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I’m happy to announce ForwardMethods.jl v1.1, a package made to help remove some boilerplate when defining forwarded/delegated methods.

Method Definition Forwarding

Concretely, suppose you have a composite type

struct A
  d::Dict{String,Any}
end

If you’re looking to delegate or forward Dict methods from an object x::A to x.d, a standard approach in Julia would be to write

for f in (:length, :isempty, :empty!)
   @eval Base.$f(x::A) = $f(getfield(x, :d))
end

which works fine if all of the functions you’re interested in forwarding have a) the same number of arguments and b) the position of x in the argument list remains constant. When forwarding a larger number of methods with a greater heterogeneity in their argument list, it can be annoying to keep track of a large number of method signatures, especially if you’d like much more control over the type signatures involved in the forwarded methods.

The @forward_methods macro allows you to avoid some of these issues by writing

@forward_methods A field=d Base.length(x::A) Base.isempty(x::A) Base.getindex(x::A, k) Base.setindex!(x::A, v, i)

or, even more compactly,

@forward_methods A field=d Base.length Base.isempty Base.getindex(_, k) Base.setindex!(x::A, v, i)

Here 0-argument expressions are implicitly expanded to a 1-argument function and _ is shorthand for x::A.

The field keyword argument supports a number of different parameters including nested dotted expressions, which forward to deeply nested structures, e.g.,

@forward_methods A field=b.c.d Base.length

will forward Base.length(x::A) to Base.length(getfield(getfield(getfield(A, :b), :c), :d)).

@forward_methods will also handle argument signatures with Type arguments, e.g.,

@forward_methods A field=b.c.d Base.eltype(::Type{A})

will define

Base.eltype(::Type{A}) = Base.eltype(fieldtype(fieldtype(fieldtype(A, :b), :c), :d))

Interface Forwarding

Similarly, the @forward_interfaces macro allows you to forward entire interfaces (i.e., named collections of particular method signatures) rather than have to write out/remember each method signature by hand.

You can also post-apply an expression to the resulting forwarded method via the map keyword argument, which allows you to do some nice fancy things like

 struct LockableDict{K,V} <: AbstractDict{K, V}
        d::Dict{K,V}
        lock::ReentrantLock
 end
 @forward_interface LockableDict{K,V} field=lock interface=lockable
 @forward_interface LockableDict{K,V} field=d interface=dict map=begin lock(_obj); try _ finally unlock(_obj) end end
 d = LockableDict(Dict{String,Int}(), ReentrantLock())

And now all of your standard dictionary calls like e.g., d["a"] = 0 will be thread-safe! (And horribly non-performant, but that’s not the point here).

This package is basically just a fancy copy+paste system but takes into account the syntax of Julia method signatures. Hopefully it makes using the Composite design pattern easier to implement for your types.

Happy forwarding!

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Version 1.3.1

Version 1.3.1 of ForwardMethods.jl is out now!

@define_interface

I’ve added a new macro for defining generic (i.e., apply to any Struct type) interfaces that don’t necessarily correspond to forwarding methods to a single field in the struct. The currently supported interfaces are

  • the getfields and setfields interfaces: Given x::T, these interfaces define $field(x) = getfield(x, $field) and $field!(x, v) = setfield!(x, $field, v) for each field in fieldnames(T), respectively. This allows you to write, for instance,
mutable struct A 
  key1::String
  key2::Int
end

@define_interface A interface=(getfields, setfields)
a = A("a", 0)
key1(a)
key1!(a, "b")
key1(a)

key2(a)
key2(a, 1)
key2(a)

Which can used to present a more easily extendable interface to your types compared to the built-in getproperty/setproperty! methods.

  • properties interface: Given x::T with struct-type subfields y1::S1, …, yn::SN, this interface defines Base.getproperty(x, k) and Base.setproperty!(x, k, v) such that when k in fieldnames(Sk), the corresponding method is forwarded to yk. This allows you to treat types composed of sub-types in a uniform way, e.g.,
julia> struct A
           key1::Int
           key2::Bool
       end

julia> struct B
           key3::String
           key4::Float64
       end

julia> struct C
           a::A
           b::B
       end

julia> @define_interface C interface=properties delegated_fields=(a,b)

julia> c = C(A(1, true), B("a", 0.0))
C(A(1, true), B("a", 0.0))

julia> (key1=c.key1, key2=c.key2, key3=c.key3, key4=c.key4, a=c.a, b=c.b)
(key1 = 1, key2 = true, key3 = "a", key4 = 0.0, a = A(1, true), b = B("a", 0.0))
  • equality interface: Given x::T, defines Base.:(==) (or Base.isequal) in the obvious way, i.e., as
Base.:(==)(x::T, y::T) = all( getfield(x,k) == getfield(y,k) for k in fieldnames(T))
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