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}}:

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()
  • 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
    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
  • 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)