mal/ps/step1_read_print.ps

/runlibfile where { pop }{ /runlibfile { run } def } ifelse % 
(types.ps) runlibfile
(reader.ps) runlibfile
(printer.ps) runlibfile

% read
/_readline { print flush (%stdin) (r) file 1024 string readline } def

/READ {
    /str exch def
    str read_str
} def


% eval
/EVAL { 2 dict begin
    % just "return" the "ast"
    /env exch def
    /ast exch def
    ast
end } def


% print
/PRINT {
    true _pr_str
} def


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

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

    { %try
        REP print (\n) print
    } stopped {
        (Error: ) print
        get_error_data false _pr_str print (\n) print
        $error /newerror false put
        $error /errorinfo null put
        clear
        cleardictstack
    } if
} bind loop

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