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This example demonstrates rule schemata.
frown List.lg
Try
expr (lexer "a (b c d) e (f g)") :: Maybe Expr
> module List
> where
> import Char
>
> data Expr = Id String
> | Call String [Expr]
> deriving (Show)
>
> type Result = Maybe
>
> %{
>
> Terminal = Ident {String}
> | "(" = LPar
> | ")" = RPar
> | "," = Comma;
>
> --test;
> --test : many (Ident {}) {is};
>
> :: expr {Expr};
> expr {Call s es} : Ident {s}, many aexpr {es};
>
> :: aexpr {Expr};
> aexpr {Id s} : Ident {s};
> {e} | "(", expr {e}, ")";
List scheme (predefined).
> {-
> :: many x {[a]} <- x {a};
> many x {[]} : ;
> {as ++ [a]} | many x {as}, 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
|
