findfirst(cond,vec){for i from 0 to length(vec) {element=vec[i];if(cond(element)){return element}}}


 : findfirst dup length 0 do I 2dup swap getindex dup 4 pick execute if nip nip nip unloop exit then drop drop loop ;

need to preserve function arity information to go to assembly
this assembly is just a sketch, not tested

% pretend I have 4 registers
% special 2 reg for top of rstack
: findfirst   findfirst:
  dup           mov(stack(0),reg(0)),
  length        mov(*reg(0),reg(0)),
  0             mov($0,reg(1))

  do
                mov(reg(1),rreg(0)) % reg(1) consumed
                mov(reg(0),rreg(1)) % reg(0) consumed     HIGHER
              loop1:
  I             mov(rreg(0),reg(0))
  2dup          mov(stack(1),reg(1))
                mov(stack(0),reg(0))
  swap          xchg(reg(1),reg(0))
  getindex      push(rreg(0))
                push(rreg(1))
                push($here)
                push(reg(0))
                push(reg(1))
                jmp(getindex)
                pop(reg(0))
                mov(stack(0),rreg(1))
                mov(stack(1),rreg(0)
                sub($16,%esp)
  dup           mov(reg(0),reg(1))
  4
  pick          mov(stack(4),reg(1))
  execute       push(rreg(0))
                push(rreg(1))
                push($here)
                push(reg(0))
                jmp(reg(1))
                pop(reg(0))
                mov(stack(0),rreg(1))
                mov(stack(1),rreg(0))
  if            jnz(reg(0),$if1)
  nip nip nip   pop(reg(0))
                sub($24,%esp)
  unloop        % on special stack registers, no need to do anything
  exit          ret
  then        if1:
                % stack position is from last if - 1
  drop drop     sub($16,%esp)

  loop          inc(rreg(0))
                mov(rreg(0),rreg(2))
                sub(rreg(1),rreg(2))
                jnz($loop1)
  ;