mal/ps/step3_env.ps

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

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

/READ {
    /str exch def
    str read_str
} def


% eval
/eval_ast { 2 dict begin
    /env exch def
    /ast exch def
    %(eval_ast: ) print ast ==
    ast _symbol? { %if symbol
        env ast env_get
    }{ ast _sequential? { %elseif list or vector
        [
            ast /data get { %forall items
                env EVAL
            } forall
        ] ast _list? { _list_from_array }{ _vector_from_array } ifelse
    }{ ast _hash_map? { %elseif list or vector
        <<
            ast /data get { %forall entries
                env EVAL
            } forall
        >> _hash_map_from_dict
    }{ % else
        ast
    } ifelse } ifelse } ifelse
end } def

/EVAL { 8 dict begin
    /env exch def
    /ast exch def

    %(EVAL: ) print ast true _pr_str print (\n) print
    ast _list? not { %if not a list
        ast env eval_ast
    }{ %else apply the list
        /a0 ast 0 _nth def
        /def! a0 eq { %if def!
            /a1 ast 1 _nth def
            /a2 ast 2 _nth def
            env a1  a2 env EVAL  env_set
        }{ /let* a0 eq { %if let*
            /a1 ast 1 _nth def
            /a2 ast 2 _nth def
            /let_env env null null env_new def
            0 2 a1 _count 1 sub { %for each pair
                /idx exch def
                let_env
                    a1 idx _nth
                    a1 idx 1 add _nth let_env EVAL
                    env_set
                    pop % discard the return value
            } for
            a2 let_env EVAL
        }{
            /el ast env eval_ast def
            el _rest el _first % stack: ast function
            exec % apply function to args
        } ifelse } ifelse
    } ifelse
end } def


% print
/PRINT {
    true _pr_str
} def


% repl
/repl_env null null null env_new def

/REP { READ repl_env EVAL PRINT } def

/_ref { repl_env 3 1 roll env_set pop } def
(+) { dup 0 _nth exch 1 _nth add } _ref
(-) { dup 0 _nth exch 1 _nth sub } _ref
(*) { dup 0 _nth exch 1 _nth mul } _ref
(/) { dup 0 _nth exch 1 _nth idiv } _ref

% 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