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
|
The standard example: arithmetic expressions (see dragon book p222).
frown --debug Expr.lg
Try
expr (lexer "a+b*c+d") :: [Expr]
Tracing the parse:
frown --debug --trace Expr.g
Try
expr (lexer "a+b*c+d")
> module Expr
> where
> import Char
>
> data AddOp = Plus | Minus
> deriving (Show)
> data MulOp = Times | Divide
> deriving (Show)
>
> data Expr = Id String
> | Add Expr AddOp Expr
> | Mul Expr MulOp Expr
> deriving (Show)
>
> type Result = []
>
> %{
>
> Terminal = Ident {String}
> | Addop {AddOp}
> | Mulop {MulOp}
> | "(" = LPar
> | ")" = RPar
> | *EOF;
>
>*expr {Expr};
> expr {Add e1 op e2} : expr {e1}, Addop {op}, term {e2};
> {e} | term {e};
>
> term {Expr};
> term {Mul e1 op e2} : term {e1}, Mulop {op}, factor {e2};
> {e} | factor {e};
>
>*factor {Expr};
> factor {e} : "(", expr {e}, ")";
> {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
|
