isless on VarName

[penelopeysm]

Sometimes, it's handy to be able to sort VarNames.

If you naively do sort(vns; by=repr) i.e. order them according to lexicographic order of their string representations, then you get things like x[10] coming before x[1].

I already actually wrote some code here to do this:

using AbstractPPL: IndexLens, PropertyLens, ComposedFunction
function Base.isless(::typeof(identity), ::Union{IndexLens,PropertyLens,ComposedFunction})
    return true
end
function Base.isless(::Union{IndexLens,PropertyLens,ComposedFunction}, ::typeof(identity))
    return false
end
Base.isless(opt1::IndexLens, opt2::PropertyLens) = true
Base.isless(opt1::PropertyLens, opt2::IndexLens) = false
function Base.isless(opt1::IndexLens, opt2::IndexLens)
    return isless(opt1.indices, opt2.indices)
end
function Base.isless(opt1::PropertyLens{sym1}, opt2::PropertyLens{sym2}) where {sym1,sym2}
    return isless(sym1, sym2)
end
function Base.isless(opt1::Union{IndexLens,PropertyLens}, opt2::ComposedFunction)
    if isequal(opt1, opt2.outer)
        return true
    else
        return isless(opt1, opt2.outer)
    end
end
function Base.isless(opt1::ComposedFunction, opt2::Union{IndexLens,PropertyLens})
    if isequal(opt1.outer, opt2)
        return false
    else
        return isless(opt1.outer, opt2)
    end
end
function Base.isless(opt1::ComposedFunction, opt2::ComposedFunction)
    if isequal(opt1.outer, opt2)
        return isless(opt1.inner, opt2.inner)
    else
        return isless(opt1.outer, opt2)
    end
end
function Base.isless(vn1::VarName{sym1}, vn2::VarName{sym2}) where {sym1,sym2}
    if sym1 == sym2
        return isless(AbstractPPL.getoptic(vn1), AbstractPPL.getoptic(vn2))
    else
        return isless(sym1, sym2)
    end
end

This was originally part of FlexiChains, but it's of course quite nasty piracy which is why I removed it. Its natural home would be AbstractPPL. And tests should be added, of course.