ps/step0_repl.ps

   1 % read
   2 /_readline { print flush (%stdin) (r) file 1024 string readline } def
   3
   4 /READ {
   5     % just "return" the input string
   6     /str exch def
   7     str
   8 } def
   9
  10
  11 % eval
  12 /EVAL {
  13     % just "return" the "ast"
  14     /env exch def
  15     /ast exch def
  16     ast
  17 } def
  18
  19
  20 % print
  21 /PRINT {
  22     % just "return" the expression
  23     /exp exch def
  24     exp
  25 } def
  26
  27
  28 % repl
  29 /REP { READ (stub env) EVAL PRINT } def
  30
  31 % repl loop
  32 { %loop
  33     (user> ) _readline
  34     not { exit } if  % exit if EOF
  35
  36     REP print (\n) print
  37 } bind loop
  38
  39 (\n) print  % final newline before exit for cleanliness
  40 quit