To install, run the following in a Julia session:

]add SubTypes

To create a custom subtype define the following data

SubType{T, P, Ctx}
        ^  ^  ^
        |  |  |
        |  |  L ______ Type Context
        |  L _________ Predicate Data
        L ____________ Underlying Type

and a predicate processing function

check_predicate(P, Val(Ctx), x::T)

which checks if x::T should be of type SubType{T, P, Ctx}. The context Ctx is the way to mark the subtypes for overloading.

The custom subtypes Constrained and ConstrainedSymbol are predefined in the SubType module. They emulate set inclusion subtyping, i.e.

x::Constrained{T, S}  <=>  x.value::T in S

The ConstrainedSymbol type emulates set inclusion for Symbol types. The Constrained type is defined as follows:

const Constrained{T,S} = SubType{T,S,:Constrained}

Inspiration for this type comes from this post by Mohamed Tarek @mohamed82008.

This module also comes with these helper functions defined for terms and types: