/usr/share/doc/frown-doc/examples/other/Prec2Expr.g is in frown-doc 0.6.1-14.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 | {-
An ambiguous grammar made unambiguous using associativity and
precedences.
frown PrecExpr.g
Try
fmap unparse $ expr (lexer "a+b+c+d") :: Maybe String
fmap unparse $ expr (lexer "a+b*c+d") :: Maybe String
fmap unparse $ expr (lexer "a+b*c*d") :: Maybe String
fmap unparse $ expr (lexer "a+b+-c+d") :: Maybe String
fmap unparse $ expr (lexer "a+-b*c+d") :: Maybe String
-}
module PrecExpr
where
import Char
import Monad
data Expr = Id String
| Negate Expr
| Plus Expr Expr
| Minus Expr Expr
| Times Expr Expr
| Divide Expr Expr
| Equal Expr Expr
deriving (Show)
unparse (Id s) = s
unparse (Negate e) = "(" ++ "-" ++ unparse e ++ ")"
unparse (Plus e1 e2) = "(" ++ unparse e1 ++ "+" ++ unparse e2 ++ ")"
unparse (Minus e1 e2) = "(" ++ unparse e1 ++ "-" ++ unparse e2 ++ ")"
unparse (Times e1 e2) = "(" ++ unparse e1 ++ "*" ++ unparse e2 ++ ")"
unparse (Divide e1 e2) = "(" ++ unparse e1 ++ "/" ++ unparse e2 ++ ")"
unparse (Equal e1 e2) = "(" ++ unparse e1 ++ "=" ++ unparse e2 ++ ")"
type Result = Maybe
%{
Terminal = IDENT {String}
| PLUS
| MINUS
| TIMES
| DIVIDE
| EQUAL
| NEGATE
| LPAR
| RPAR;
nonassoc 4 EQUAL;
left 6 PLUS;
left 6 MINUS;
left 7 TIMES;
left 7 DIVIDE;
nonassoc 8 NEGATE;
Nonterminal = *expr {Expr};
expr {Negate e} : MINUS, expr {e}, prec NEGATE;
{Plus e1 e2} | expr {e1}, PLUS, expr {e2};
{Minus e1 e2} | expr {e1}, MINUS, expr {e2};
{Times e1 e2} | expr {e1}, TIMES, expr {e2};
{Divide e1 e2} | expr {e1}, DIVIDE, expr {e2};
{Equal e1 e2} | expr {e1}, EQUAL, expr {e2};
{e} | LPAR, expr {e}, RPAR;
{Id s} | IDENT {s};
}%
frown ts = fail "syntax error"
data Terminal = IDENT String | PLUS | MINUS | TIMES | DIVIDE | EQUAL | LPAR | RPAR
lexer :: String -> [Terminal]
lexer [] = []
lexer ('+' : cs) = PLUS : lexer cs
lexer ('-' : cs) = MINUS : lexer cs
lexer ('*' : cs) = TIMES : lexer cs
lexer ('/' : cs) = DIVIDE : lexer cs
lexer ('=' : cs) = EQUAL : lexer cs
lexer ('(' : cs) = LPAR : lexer cs
lexer (')' : cs) = RPAR : lexer cs
lexer (c : cs)
| isAlpha c = let (n, cs') = span isAlphaNum cs
in IDENT (c : n) : lexer cs'
| otherwise = lexer cs
|