mal/ps/step0_repl.ps

% read
/_readline { print flush (%stdin) (r) file 99 string readline } def

/READ {
    % just "return" the input string
    /str exch def
    str
} def


% eval
/EVAL {
    % just "return" the "ast"
    /env exch def
    /ast exch def
    ast
} def


% print
/PRINT {
    % just "return" the expression
    /exp exch def
    exp
} def


% repl
/REP { READ (stub env) EVAL PRINT } def

% repl loop
{ %loop
    (user> ) _readline
    not { exit } if  % exit if EOF

    REP print (\n) print
} bind loop

(\n) print  % final newline before exit for cleanliness
quit