Operator-sensitive promotion¶

In certain cases, the simple rules for promotion may not be sufficient. For example, consider a type that can represent an object with physical units, here restricted to a single unit like “meter”:

immutableMeterUnits{T,P}<:Numberval::TendMeterUnits{T}(val::T,pow::Int)=MeterUnits{T,pow}(val)m=MeterUnits(1.0,1)# 1.0 meter, i.e. units of lengthm2=MeterUnits(1.0,2)# 1.0 meter^2, i.e. units of area

Now let’s define the operations + and * for these objects: m+m should have the type of m but m*m should have the type of m2. When the result type depends on the operation, and not just the input types, promote_rule will be inadequate.

Fortunately, it’s possible to provide such definitions via promote_op:

Base.promote_op{R,S}(::Base.AddFun,::Type{MeterUnits{R,1}},::Type{MeterUnits{S,1}})=MeterUnits{promote_type(R,S),1}Base.promote_op{R,S}(::Base.MulFun,::Type{MeterUnits{R,1}},::Type{MeterUnits{S,1}})=MeterUnits{promote_type(R,S),2}Base.promote_op{R,S}(::Base.DotMulFun,::Type{MeterUnits{R,1}},::Type{MeterUnits{S,1}})=MeterUnits{promote_type(R,S),2}

The first one defines the promotion rule for +, and the second one for *. AddFun, MulFun, and DotMulFun are “functor types” defined in functor.jl.

It’s worth noting that as julia’s internal representation of functions evolves, this interface may change in a future version of Julia.