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|
{-
This grammar illustrates overlapping patterns (terminals). Note that
the patterns are ordered by generality (most general one last).
frown Overlapping
-}
module Overlapping
where
import Char
data Expr = Var String
| If Expr Expr Expr
deriving (Show)
type Result = Maybe
frown ts = fail "syntax error"
%{
Terminal = "if" = Ident "if"
| "then" = Ident "then"
| "else" = Ident "else"
| Ident {String};
Nonterminal = expr {Expr};
expr {Var s} : Ident {s};
{If e1 e2 e3} | "if", expr {e1}, "then", expr {e2}, "else", expr {e3};
}%
data Terminal = Ident String
deriving (Show)
lexer :: String -> [Terminal]
lexer [] = []
lexer (c : cs)
| isAlpha c = let (n, cs') = span isAlphaNum cs
in Ident (c : n) : lexer cs'
| otherwise = lexer cs
|
