mal/ps/step3_env.ps

(types.ps) run
(reader.ps) run

% read
/READ {
    /str exch def
    str read_str
} def


% eval
/eval_ast { 2 dict begin
    /env exch def
    /ast exch def
    %(eval_ast: ) print ast ==
    /nametype ast type eq { %if symbol
        env ast env_get
    }{ /arraytype ast type eq { %elseif list
        [
            ast {
                env EVAL
            } forall
        ]
    }{ % else
        ast
    } ifelse } ifelse
end } def

/EVAL { 10 dict begin
    /env exch def
    /ast exch def
    %(EVAL: ) print ast ==
    /arraytype ast type ne { %if not a list
        ast env eval_ast
    }{ %else apply the list
        /a0 ast 0 get def
        /def! a0 eq { %if def!
            /a1 ast 1 get def
            /a2 ast 2 get def
            env a1  a2 env EVAL  env_set
        }{ /let* a0 eq { %if let*
            /a1 ast 1 get def
            /a2 ast 2 get def
            /let_env env [ ] [ ] env_new def
            0 2 a1 length 1 sub { %for each pair
                /idx exch def
                let_env
                    a1 idx get
                    a1 idx 1 add get let_env EVAL
                    env_set
            } for
            a2 let_env EVAL
        }{
            /el ast env eval_ast def
            el _rest % args array
            el _first cvx % function
            %(vvv\n) print pstack (^^^\n) print
            exec % apply function to args
        } ifelse } ifelse
    } ifelse
end } def


% print
/PRINT {
    true _pr_str
} def


% repl
/repl_env null [ ] [ ] env_new def

/REP { READ repl_env EVAL PRINT } def
/_ref { repl_env 3 1 roll env_set pop } def

(+) { dup 0 get exch 1 get add } _ref
(-) { dup 0 get exch 1 get sub } _ref
(*) { dup 0 get exch 1 get mul } _ref
(/) { dup 0 get exch 1 get idiv } _ref

/stdin (%stdin) (r) file def 

{ % loop
    (user> ) print flush

    %(%lineedit) (r) file 99 string readline
    stdin 99 string readline

    not { exit } if  % exit if EOF

    %(\ngot line: ) print dup print (\n) print flush

    { %try
        REP print (\n) print
    } stopped {
        (Error: ) print
        get_error_data false _pr_str print (\n) print
        clear
    } if
} bind loop

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