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
|
{-
Variation of `Loop1.g' that terminates (because look-ahead information
avoids entering the loop, the look-ahead is empty).
frown --debug Loop2.g
Try
s "$"
-}
module Loop
where
import Monad
instance MonadPlus IO where
mzero = fail "mzero"
m `mplus` n = putStrLn "** choice" >> m >> putStrLn "** backtrack" >> n
frown ts = putStrLn "*** syntax error" >> return undefined
type Terminal = Char
type Result = IO
{-
frown _ = fail "syntax error"
-}
%{
Terminal = 'a' | 'b' | 'c' | *'$';
Nonterminal = s | x | y;
s : x;
| 'a';
x : y, x, 'c';
y : ;
}%
|
