singular_ring not working for finite prime fields

[thofma]

julia> Fx, = GF(fmpz(3))["x", "y"]
(Multivariate Polynomial Ring in x, y over Galois field with characteristic 3, AbstractAlgebra.Generic.MPoly{gfp_fmpz_elem}[x, y])

julia> Oscar.singular_ring(Fx)
ERROR: pointer_from_objref cannot be used on immutable objects
Stacktrace:
  [1] error(s::String)
[...]

@tthsqe12

[tthsqe12]

Well, you can't use n_unknown on immutables.
This is a well-known fact. :)

Oscar.jl/experimental/Rings/QabAndPChars.jl

Lines 23 to 25 in c4eca1e

	#singular cannot use immutable structs
	mutable struct QabField <: Nemo.Field # union of cyclotomic fields
	end

Unfortunately, singular offers no native analogue of gfp_fmpz_elem, so either we make gfp_fmpz_elem mutable, or make a mutable wrapper of it?

[thofma]

How about not creating an n_unkown? Or is there no corresponding Singular type?

[tthsqe12]

Unfortunately, singular offers no native analogue of gfp_fmpz_elem

[thofma]

What about the ResidueRing trick from here: #528?

[tthsqe12]

No - then Singular doesn't know it is a field.

[tthsqe12]

This is now fixed as:

julia> Fx, = GF(fmpz(3))["x", "y"]
(Multivariate Polynomial Ring in x, y over Galois field with characteristic 3, AbstractAlgebra.Generic.MPoly{gfp_fmpz_elem}[x, y])

julia> Oscar.singular_ring(Fx)
Singular Polynomial Ring (Coeffs(17)),(x,y),(lp(2),C)

julia> typeof(gen(ans, 1))
Singular.spoly{Singular.n_FieldElem{Singular.FieldElemWrapper{Nemo.GaloisFmpzField, gfp_fmpz_elem}}}