/usr/share/doc/frown-doc/examples/other/Loop.g is in frown-doc 0.6.1-14.
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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 45 46 47 48 49 50 51 52 | {-
Example for an ambiguous grammar that causes the parser to loop on
backtracking (hidden left recursion).
frown --debug Loop.g
Try
x "bacc$" :: Maybe ()
x "bacc$" :: [()]
Using
frown --debug --trace Loop.g
shows why. Try
x "bacc$"
-}
module Loop
where
import Monad
instance MonadPlus IO where
mzero = fail "mzero"
m `mplus` n = putStrLn "** choice" >> m >> putStrLn "** backtrack" >> n
frown ts = putStrLn "*** syntax error" >> return undefined
type Terminal = Char
type Result = IO
{-
frown _ = fail "syntax error"
-}
%{
Terminal = 'a' | 'b' | 'c' | *'$';
Nonterminal = x | y;
x : y, x, 'c';
| 'a';
y : 'b';
| ;
}%
|