promote_rule example

promote_rule

julia> import Base: promote_rule, convert

julia> struct USD 
         value
       end

julia> struct Painting
         rating
       end 

julia> promote_rule(::Type{USD}, ::Type{Painting}) = USD
promote_rule (generic function with 138 methods)

julia> promote_type(Painting, USD)
USD

julia> convert(::Type{USD}, p::Painting) = USD(p.rating * 1000)
convert (generic function with 187 methods)

julia> mona_lisa = Painting(250)
Painting(250)

julia> grab(x::USD, y::USD) = USD(x.value + y.value)
grab (generic function with 2 methods)

julia> grab(x::USD, y::Painting) = grab(promote(x, y)...)
grab (generic function with 2 methods)

julia> grab(USD(100), mona_lisa)
USD(250100)

julia> grab(x::Painting, y::USD) = grab(promote(x, y)...)
grab (generic function with 4 methods)

julia> grab(mona_lisa, USD(100))
USD(250100)