ConstructFrom.jl

Construct objects without spelling out their types if the types can be inferred from context.

julia> vec::Vector{Vector{Int}} = construct_from(undef, 3);

julia> vec
3-element Vector{Vector{Int64}}:
 #undef
 #undef
 #undef

construct_from() may be useful in a number of circumstances.

• When defining default values for function arguments.

  foo(data::Vector{Int} = construct_from()) = ...

• When defining default values for fields of @kwdef structs.

  Base.@kwdef struct Foo
      data::Vector{Int} = construct_from()
  end

• When initializing typed globals.

  data::Vector{Int} = construct_from()

• When providing a default value for dictionary lookups.

  dict = Dict{Int, Vector{Int}}()
  get(dict, 42, construct_from())

---

Note. construct_from() exploits that Julia inserts an implicit convert() in many circumstances. (Most prominently, a::T = b is syntactic sugar for a = typeassert(convert(T, b), T).) Correspondingly, construct_from() fails in cases where there is no conversion, or the conversion happens too late.

• No conversion:

  julia> foo(data::Vector{Int}) = data;
  
  julia> # Doesn't work because no conversion
         foo(construct_from())
  ERROR: MethodError: no method matching foo(::ConstructFrom.Arguments{...})
  
  julia> # Let's add the conversion ourselves
         foo(data) = foo(convert(Vector{Int}, data));
  
  julia> # Now it works
         foo(construct_from())
  Int64[]

• Conversion happens too late:

  julia> a = construct_from()
         push!(a, 42) # <- Julia won't deduce the type of `a` from how it's used later
         b::Vector{Int} = a
  ERROR: MethodError: no method matching push!(::ConstructFrom.Arguments{...}, ::Int64)