This example demonstrates rule schemata.

	frown Schemes.lg

Try

	expr (lexer "a (b c, d) e (f, g)") :: Maybe Expr

> module Schemes
> 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 {Call s es}              :  Ident {s}, many aexpr {es};
>
> :: aexpr {Expr};
> aexpr {Id s}                  :  Ident {s};
>       {Tuple es}              |  "(", sepBy expr comma {es}, ")";
>
> :: comma;
> comma                         :  ",";

Schemes (predefined).

> {-
> :: many x {[a]}               <- x {a};
> many x {s []}                 :  many' x {s};
>
> :: many' x {[a] -> [a]}       <- x {a};
> many' x {\ as -> as}          :  ;
>         {\ as -> s (a : as)}  |  many' x {s}, x {a};
>
> :: 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 {[a]}            :  x {a};
>              {a : as}         |  x {a}, sep, sepBy1 x sep {as};
> -}

> }%
>
> 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