PolynomialRing.jl

# BEFORE:
function PolynomialRing{T <: Ring}(::Type{T}, s::String)
   S = symbol(s)
   T1 = Poly{T, S}
   T2 = T
   
   # Conversions and promotions
   eval(:(Base.convert(::Type{$T1}, x::$T) = Poly($T1, [x])))
   eval(:(Base.promote_rule(::Type{$T1}, ::Type{$T}) = $T1))

   P = T2.parameters
   while length(P) > 0
      T2 = P[1]
      if isa(T2, DataType) && T2 <: Ring
         eval(:(Base.convert(::Type{$T1}, x::$T2) = Poly($T1, [convert($T, x)])))
         eval(:(Base.promote_rule(::Type{$T1}, ::Type{$T2}) = $T1))
         P = T2.parameters
      else
         break
      end
   end

   if T != ZZ
      eval(:(Base.convert(::Type{$T1}, x::ZZ) = Poly($T1, [convert($T, x)])))
      eval(:(Base.promote_rule(::Type{$T1}, ::Type{ZZ}) = $T1))
   end
   eval(:(Base.convert(::Type{$T1}, x::Integer) = Poly($T1, [convert($T, x)])))
   eval(:(Base.promote_rule{R <: Integer}(::Type{$T1}, ::Type{R}) = $T1))

   # (Type, gen) 
   return (Poly{T, S}, Poly(Poly{T, S}, [T(0), T(1)]))
end
-----------
#AFTER: 
Base.eltype{T}(::Type{Poly{T}}) = T
Base.convert{T<:Ring, S}(::Type{Poly{T,S}}, x::Ring) = Poly{T,S}([convert(T,x)])
Base.promote_rule{T1<:Poly, T<:Ring}(::Type{T1}, ::Type{T}) = T1

Base.convert{T1<:Poly}(::Type{T1}, x::Integer) = Poly(T1, [convert(eltype(T1), x)])
Base.promote_rule{T1<:Poly, R <: Integer}(::Type{T1}, ::Type{R}) = T1

PolynomialRing{T <: Ring}(::Type{T}, s::String) = PolynomialRing(T, symbol(s))
function PolynomialRing{T <: Ring}(::Type{T}, S::Symbol)
   return (Poly{T, S}, Poly(Poly{T, S}, [T(0), T(1)]))
end