Crenshaw's Compiler Tutorial in VimL

let s:look = ''
let s:input = ''
let s:output = ''
let s:pos = 0

function! GetChar()
  let s:look = strpart(s:input, s:pos, 1)
  let s:pos += 1
endfunction

function! Error(msg)
  echoerr "Error: " . a:msg . "."
endfunction

function! Abort(msg)
  call Error(a:msg)
  " Halt   let s:running = 0
endfunction

function! Expected(msg)
  call Abort(a:msg . ' Expected')
endfunction

function! Match(c)
  if s:look == a:c
    call GetChar()
  else
    call Expected("'" . x . "'")
  endif
endfunction

function! IsAddop(c)
  return index(['+', '-'], a:c) != -1
endfunction

function! IsAlpha(c)
  return match(a:c, '\C[A-Z]') != -1
endfunction

function! IsDigit(c)
  return match(a:c, '[0-9]') != -1
endfunction

function! GetName()
  if !IsAlpha(s:look)
    call Expected('Name')
  endif
  let retval = toupper(s:look)
  call GetChar()
  return retval
endfunction

function! GetNum()
  if !IsDigit(s:look)
    call Expected('Integer')
  endif
  let retval = s:look
  call GetChar()
  return retval
endfunction

function! Emit(code)
  let s:output .= a:code
endfunction

function! EmitLn(code)
  call Emit(a:code)
  let s:output .= "\r"
endfunction

function! Init(input)
  let s:look = ''
  let s:input = a:input
  let s:output = ''
  let s:pos = 0
  call GetChar()
endfunction

function! Ident()
  let name = GetName()
  if s:look == '('
    call Match('(')
    call Match(')')
    call EmitLn('BSR ' . name)
  else
    call EmitLn('MOVE ' . name . '(PC),D0')
  endif
endfunction

function! Factor()
  if s:look == '('
    call Match('(')
    call Expression()
    call Match(')')
  elseif IsAlpha(s:look)
    call Ident()
  else
    call EmitLn('MOVE #' . GetNum() . ',D0')
  endif
endfunction

function! Multiply()
  call Match('*')
  call Factor()
  call EmitLn('MULS (SP)+,D0')
endfunction

function! Divide()
  call Match('/')
  call Factor()
  call EmitLn('MOVE (SP)+,D1')
  call EmitLn('DIVS D1,D0')
endfunction

function! Term()
  call Factor()
  while index(['*', '/'], s:look) != -1
    call EmitLn('MOVE D0,-(SP)')
    if s:look == '*'
      call Multiply()
    elseif s:look == '/'
      call Divide()
    endif
  endwhile
endfunction

function! Add()
  call Match('+')
  call Term()
  call EmitLn('ADD (SP)+,D0')
endfunction

function! Subtract()
  call Match('-')
  call Term()
  call EmitLn('SUB (SP)+,D0')
  call EmitLn('NEG D0')
endfunction

function! Expression()
  if IsAddop(s:look)
    call EmitLn('CLR D0')
  else
    call Term()
    while IsAddop(s:look)
      call EmitLn('MOVE D0,-(SP)')
      if s:look == '+'
        call Add()
      elseif s:look == '-'
        call Subtract()
      endif
    endwhile
  endif
endfunction

function! Assignment()
  let name = GetName()
  call Match('=')
  call Expression()
  call EmitLn('LEA ' . name . '(PC),A0')
  call EmitLn('MOVE D0,(A0)')
endfunction

function! Parse(input)
  call Init(a:input)
  call Expression()
  if s:look != "\n"
    call Expected('Newline')
  else
    echomsg s:output
  endif
endfunction

call Parse("(1+2)/9\n")