AST-fun with julia

algebra.jl

function simplify(e::Expr)
  if e.head == :call
    if e.args[1] == :+
      simplify_add(e)
    end
  end
end

function simplify_add(e::Expr)
  terms = Dict{Symbol, Int}()
  for idx in 2:length(e.args)
    if typeof(e.args[idx]) <: Integer
      terms[:_] =  haskey(terms, :_) ? e.args[idx] + 1 : 1
    elseif typeof(e.args[idx]) <: Symbol
      terms[e.args[idx]] = haskey(terms, e.args[idx]) ? terms[e.args[idx]] += 1 : 1
    end
  end
  #next, collate the values.
  splice!(e.args, 2:length(e.args), [expr_mult(terms[s], s) for s in keys(terms)])
  e
end

expr_mult(v, s::Symbol) = s == :_ ? v : :($v * $s)

simplify_add(:(x + 1 + 3 + x))
==> 4 + 2x