gistfile1.txt

/switch { % key cases otherwise <switch> any
  2 index null eq {
    exch pop exch pop
  } {
    1 index 3 index known {
      pop exch get
    } {
      exch pop exch pop
    } ifelse
  } ifelse
} def

/ifopt { % ?any proc proc <ifopt> -
  3 2 roll dup null eq { pop exch pop exec } { exch pop exch exec } ifelse
} def

/readc { dup read not { null } if } def

/digit { % file ?char <digit> file ?char ?digit
  48 1 index le 1 index 57 le and { 48 sub exch readc 3 2 roll } { null } ifelse
} def

/skipspaces { % file ?char <skipspaces> file ?char
  { % file ?char self
    exch { % file self char
      dup 10 eq 1 index 32 eq or {
        pop [ 3 2 roll readc 4 3 roll dup /exec cvx ] cvx exec
      } { exch pop } ifelse
    } { % file self
      pop null
    } ifopt
  } dup exec
} def

/gexpr { % child opdict file ?char <gexpr> file ?char ?result
  3 index exec {
    { % child opdict file ?char expr self
      4 2 roll skipspaces % child opdict expr self file ?char
      dup 5 index null switch % child opdict expr self file ?char ?op
      {
        exch pop exch readc % child opdict expr self op file ?char
        6 index exec { % child opdict expr self op file ?char expr
          [ 7 6 roll {} /forall cvx 5 4 roll {} /forall cvx 10 9 roll ] cvx
          [ exch exec cvx ] 4 3 roll dup exec
        } { 7 5 roll pop pop pop pop pop null } ifopt
      } { % child opdict expr self file ?char
        3 2 roll pop 5 3 roll pop pop 3 2 roll
      } ifopt
    } dup exec
  } { % child opdict file ?char
    4 2 roll pop pop null
  } ifopt
} def

/expr {
  { { factor } << 42 /mul 47 /div >> 4 2 roll gexpr } % 42 = '*', 47 = '/'
  << 43 /add 45 /sub >> % 43 = '+', 45 = '-'
  4 2 roll gexpr
} def

/factor { % file ?char <factor> file ?char ?result
  skipspaces {
    dup 40 eq {
      pop readc expr 3 1 roll skipspaces % ?result file ?char
      dup 41 eq { pop readc 3 2 roll } { 3 2 roll pop null } ifelse
    } { % file ?char
      digit { % file ?char acc
        { % file ?char acc self
          4 2 roll digit { % acc self file ?char digit
            5 4 roll 10 mul add % self file ?char acc
            4 3 roll dup exec
          } {
            4 2 roll pop [ exch ]
          } ifopt
        } dup exec
      } { null } ifopt
    } ifelse
  } { null null } ifopt
} def

(%stdin) (r) file
readc expr cvx { dup == exec = } { (error) = } ifopt