Remove additional rule that parameters must match for dispatch

General Usage
1

https://github.com/JuliaLang/julia/pull/23117

Can someone tell what this change brings?

Can you show me an example what it enabled?

Thanks

2

In Julia v0.6, this doesn’t work:

julia> f(x::Union{Symbol, Vector{T}}) where {T} = "hello"
f (generic function with 1 method)

julia> f(:foo)
ERROR: MethodError: no method matching f(::Symbol)
Closest candidates are:
  f(::Union{Array{T,1}, Symbol}) where T at REPL[1]:1

because Julia v0.6 required that the parameter T must be defined by the type of the function arguments (in this case, x::Symbol). . When you pass a Symbol as x, though, T is not defined and the method doesn’t match, even though it should work just fine for a Symbol argument.

In Julia v0.7 this requirement is removed:

julia> f(x::Union{Symbol, Vector{T}}) where {T} = "hello"
f (generic function with 1 method)

julia> f(:foo)
"hello"

and the function will work, as long as you don’t actually use the value of T (since it’s still not defined when x is a Symbol).

4 Likes
3

FWIW I’ve run into this limitation on a real project. See Understanding the use of Union with parametric types for a brief discussion. I didn’t know the issue had been addressed. That’s cool!