; useful constants
make "false 1=0
make "true  1=1
make "lf char 10
make "sp char 32

; non-digits legal in expression
make "operators (lput sp [+ - * / \( \)])

; display help message
to show_help :digits
  type lf
  print sentence quoted [Using only these digits:] :digits
  print sentence quoted [and these operators:] [* / + -]
  print quoted [\(and parentheses as needed\),]
  print quoted [enter an arithmetic expression
     which evaluates to exactly 24.]
  type lf
  print quoted [Enter \"!\" to get fresh numbers.]
  print quoted [Enter \"q\" to quit.]
  type lf
end

make "digits []
make "done false
until [done] [

  if empty? digits [
    make "digits (map [(random 9) + 1] [1 2 3 4])
  ]

  (type "Solution sp "for sp digits "? sp )
  make "expression readrawline

  ifelse [expression = "?] [

    show_help digits

  ] [ifelse [expression = "q] [

    print "Bye!
    make "done true

  ] [ifelse [expression = "!] [

    make "digits []

  ] [
    make "exprchars ` expression
    make "allowed (sentence digits operators)

    ifelse (member? false (map [[c] member? c allowed] exprchars)) [
      (print quoted [Illegal character in input.])
    ] [
      catch "error [
        make "syntax_error true
        make "testval (run expression)
        make "syntax_error false
      ]
      ifelse syntax_error [
        (print quoted [Invalid expression.])
      ] [
        ifelse (testval = 24) [
          print quoted [You win!]
          make "done true
        ] [
          (print (sentence
            quoted [Incorrect \(got ] testval quoted [instead of 24\).]))
        ]
      ]
    ]
  ]]]
]
bye