ps/step0_repl.ps


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44

% read
/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

/stdin (%stdin) (r) file def 

{ % loop
    (user> ) print flush

    stdin 99 string readline

    not { exit } if  % exit if EOF

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

    REP print (\n) print
} bind loop

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