When I last wrote Julia I think it was unclear to me how subtyping worked with type parameters, or writing type specifiers for methods was complicated by them.

When I last wrote Julia I think it was unclear to me how subtyping worked with type parameters, or writing type specifiers for methods was complicated by them. I think all type constructors are invariant, so you couldn’t have T{X} < T{Y} unless X = Y (but I think you can have a rule like S{X} < T{X}). This makes sense in some cases as covariance or contravariance don’t work for arrays (suppose I want an Animal array but you give me a Dog array, then when I read elements I get Animals as expected but if I were to try to write a Cat there, it would be bad. Java has this rule and will throw a runtime extension in the case described), but it can work for immutable types, though Julia can’t enforce that all subtypes are immutable and type parameters needn’t be types anyway.