supertypes of minkowski and mahalanobis do not match most other classes

[CMGreenspon]

julia> supertypes(typeof(euclidean))
(Euclidean, Distances.UnionMetric, Metric, SemiMetric, PreMetric, Any)

julia> supertypes(typeof(minkowski))
(typeof(minkowski), Function, Any)

julia> supertypes(typeof(mahalanobis))
(typeof(mahalanobis), Function, Any)

Easy to get around this when trying to quickly switch between distance metrics but it seems like odd behavior

[richardreeve]

I think the fact that most of the shorthand distance functions (like euclidean()) give supertypes at all is a "feature" of how the functions are defined here:

Distances.jl/src/metrics.jl

Line 328 in 6d0110d

@eval @inline (dist::$M)(a, b) = _evaluate(dist, a, b, parameters(dist))

The actual type hierarchy can be derived from the types themselves, which are capitalised as Euclidean, Mahalanobis and Minkowski, etc:

julia> supertypes(Euclidean)
(Euclidean, Distances.UnionMetric, Metric, SemiMetric, PreMetric, Any)

julia> supertypes(Mahalanobis)
(Mahalanobis, Metric, SemiMetric, PreMetric, Any)

julia> supertypes(Minkowski)
(Minkowski, Distances.UnionMetric, Metric, SemiMetric, PreMetric, Any)