How to represent a performant heterogenous stack in Julia

I would like to implement a simple concatenative language (aka Joy or Factor) as a DSL in Julia and I am troubled how to optimally represent the stack.

The stack, which represents both data and program code, should be able to hold a sequence of items of different types. In the simplest case Ints, Symbols and, recursively again, stacks (to represent quoted code). The program will then heavily use push! and pop! to shuffle values between different such stacks.

One obvious implementation in Julia, which works but runs rather slow, is to use cell arrays. For example, the following Joy stack [ 1 [ 1 2 +] i + ] (which evaluates to [4]) can be implemented in Julia as stack = Any[:+,:i,Any[:+,2,1],1]. My typical code then looks like this:

x = pop!(callstack)
if isa(x,Int)
   push!(x,datastack)
elseif isa(x,Symbol)
   do_stuff(x,datastack)
end

This, however, runs really slow and uses huge memory allocations, probably because such code is not typestable (which is a big performance bottleneck in Julia).

Using C, I would represent the stack compactly as an array (or alternatively as a linked list) of a union:

typedef union Stackelem{
    int val;
    char *sym;
    union Stackelem *quote;
} Stackelem;

Stackelem stack[n];

But how can I achieve such a compact representation of the heterogeneous stack in Julia, and how I avoid the type instability?

Solution

This is one way, another way would be to represent args with type Vector{Any}:

julia> immutable Exp
          head::Symbol
          args::Tuple
       end

julia> q = Exp(:+, (1, Exp(:-, (3, 4))))
Exp(:+,(1,Exp(:-,(3,4))))

edit: Another way to represent it might be:

immutable QuoteExp{T} ; vec::Vector{T} ; end
typealias ExpTyp Union{QuoteExp, Int, Symbol}
typealias Exp QuoteExp{ExpTyp}

and then you can do the following:

julia> x = Exp(ExpTyp[:+, 1, 2])
QuoteExp{Union{Int64,QuoteExp{T},Symbol}}(Union{Int64,QuoteExp{T},Symbol}[:+,1,2])
julia> x.vec[1]
:+
julia> x.vec[2]
1
julia> x.vec[3]
2
julia> push!(x.vec,:Scott)
4-element Array{Union{Int64,QuoteExp{T},Symbol},1}:
  :+    
 1      
 2      
  :Scott
julia> x.vec[4]
:Scott