I defined a new type (
Mod29 – integers mod 29).
struct Mod29 <: Real value::Int8 function Mod29(value::Integer) new(Int8(mod(value, 29))) end end
In another, generic, algorithm, I need to know whether comparison is defined on a type. I did not define comparison on
Mod29, and indeed
Mod29(1) < Mod29(3) fails:
**julia>** Mod29(1) < Mod29(3) **ERROR:** < not defined for Mod29 Stacktrace:  **error(** ::String, ::String, ::Type **)** at **./error.jl:42**  **no_op_err(** ::String, ::Type **)** at **./promotion.jl:410**  **<(** ::Mod29, ::Mod29 **)** at **./promotion.jl:426**  top-level scope at **none:0**
hasmethod(<, (Mod29, Mod29)) and
hasmethod(isless, (Mod29, Mod29)) both return
I see that
isless fail trying to do promotion, but I don’t see how to tell that ‘<’ will fail, without trying it and catching the error.
For your amusement, the algorithm I need to change is LU decomposition, where it tries to pivot on the max. If comparison isn’t defined, I just want it to find an element with an inverse, to pivot on. I want to be able to take inverses of matrices of modular numbers.