Package ambiguity

[RomainBrault]

Hello,

There is a ambiguity with the package measure.jl:

WARNING: New definition 
    *(#T<:Any, Type{Measures.Length{:cx, T<:Any}}) at /Users/Romain/.julia/v0.4/Compose/src/measure.jl:13
is ambiguous with: 
    *(IterativeSolvers.AbstractMatrixFcn, Any) at /Users/Romain/.julia/v0.4/IterativeSolvers/src/common.jl:118.
To fix, define 
    *(_<:IterativeSolvers.AbstractMatrixFcn, Type{Measures.Length{:cx, T<:Any}})
before the new definition.
WARNING: New definition 
    *(#T<:Any, Type{Measures.Length{:cy, T<:Any}}) at /Users/Romain/.julia/v0.4/Compose/src/measure.jl:14
is ambiguous with: 
    *(IterativeSolvers.AbstractMatrixFcn, Any) at /Users/Romain/.julia/v0.4/IterativeSolvers/src/common.jl:118.
To fix, define 
    *(_<:IterativeSolvers.AbstractMatrixFcn, Type{Measures.Length{:cy, T<:Any}})

Br,

Romain.

[jiahao]

Unfortunately I don't think it's possible to fix this problem in this current version of Julia. IterativeSolvers cannot express the required interface of the linear operator as a type constraint within the current type system. Instead, we would require a mechanism like traits (JuliaLang/julia#13222) or interfaces (JuliaLang/julia#6975), either of which would be a worthy software design experiment to try out.

Another possibility that has been discussed is to turn ambiguity warnings into run time errors (c.f. JuliaLang/julia#6190), which would only affect users that tried to call ambiguous methods.