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
|
This example demonstrates rule schemata.
frown Tuple.lg
Try
expr (lexer "(a, b, (c, d), ((e, f), g))") :: Maybe Expr
> module Tuple
> where
> import Char
>
> data Expr = Id String
> | Call String [Expr]
> | Tuple [Expr]
> deriving (Show)
>
> type Result = Maybe
>
> %{
>
> Terminal = Ident {String}
> | "(" = LPar
> | ")" = RPar
> | "," = Comma;
>
> :: expr {Expr};
> expr {Id s} : Ident {s};
> {Tuple es} | "(", sepBy expr "," {es}, ")";
Schemes (predefined).
> {-
> sepBy x sep {[a]} <- x {a}, sep;
> sepBy x sep {[]} : ;
> {as} | sepBy1 x sep {as};
>
> sepBy1 x sep {[a]} <- x {a}, sep;
> sepBy1 x sep {s []} : sepBy1' x sep {s};
>
> sepBy1' x sep {[a] -> [a]} <- x {a}, sep;
> sepBy1' x sep
> {\ as -> a : as} : x {a};
> {\ as -> s (a : as)} | sepBy1' x sep {s}, sep, x {a};
> -}
> }%
>
> frown ts = fail "syntax error"
>
> data Terminal = Ident String | LPar | RPar | Comma
> deriving (Show)
>
> lexer :: String -> [Terminal]
> lexer [] = []
> lexer ('(' : cs) = LPar : lexer cs
> lexer (')' : cs) = RPar : lexer cs
> lexer (',' : cs) = Comma : lexer cs
> lexer (c : cs)
> | isAlpha c = let (n, cs') = span isAlphaNum cs
> in Ident (c : n) : lexer cs'
> | otherwise = lexer cs
|
