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
|
{-
The standard example: arithmetic expressions (see dragon book p222).
frown --debug Test.g
Try
expr (lexer "a+b*c+d") :: [Expr]
Tracing the parse:
frown --debug --trace Expr.g
Try
expr (lexer "a+b*c+d")
-}
module Test
where
import Char
data AddOp = Plus | Minus
deriving (Show)
data MulOp = Times | Divide
deriving (Show)
data Expr = Add Expr AddOp Expr
| Mul Expr MulOp Expr
| Id String
deriving (Show)
type Result = []
%{
Terminal = IDENT {String}
| ADDOP {AddOp}
| MULOP {MulOp}
| LPAR
| RPAR
| *EOF;
Nonterminal = *expr {Expr}
| term {Expr}
| factor {Expr};
expr {Add e1 op e2} : expr {e1}, ADDOP {op}, term {e2};
{e} | term {e};
term {Mul e1 op e2} : term {e1}, MULOP {op}, factor {e2};
{e} | factor {e};
factor {e} : LPAR, expr {e}, RPAR;
{Id s} | IDENT {s};
}%
frown ts = fail "syntax error"
data Terminal = IDENT String | ADDOP AddOp | MULOP MulOp | LPAR | RPAR | EOF
deriving (Show)
lexer :: String -> [Terminal]
lexer [] = [EOF]
lexer ('+' : cs) = ADDOP Plus : lexer cs
lexer ('-' : cs) = ADDOP Minus : lexer cs
lexer ('*' : cs) = MULOP Times : lexer cs
lexer ('/' : cs) = MULOP Divide : 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
|
