recursive type parameter constraints about julia

[kimikage]

BTW, I think the typical and practical workaround for this is to use abstract types.

abstract type AbstractS{A} end

struct S{A,B<:Union{Nothing,AbstractS{A}}} <: AbstractS{A}
    a::A
    b::B
end

I think it is somewhat misleading to bring up the S{A,B} without constraints.

[nsajko]

Perhaps an OK way to get better type inference is to make the instance properties private, and define a getter with appropriate type assertions. Something like:

function list_tail(l::S)
    r = l.b
    r::Union{Nothing,S{typeof(l.a)}}
end

After applying @kimikage's workaround below, the getter could be simplified to:

function list_tail(l::S)
    r = l.b
    r::Union{Nothing,S}
end

[nsajko]

This is pretty nice, thank you very much, though not perfect on its own. The subtyping in your solution guarantees that, for any s isa S, the element type of s.b is the same as the element type of s if s.b isa S.

When combined with my getter-with-type-assertions solution above, I suppose this should be able to convince type inference of the homogeneity, though.