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|
{-
This example illustrates multiple entry point.
frown Stat.g
Try
stat (lexer "a=b") :: IO Stat
expr (lexer "a") :: IO Expr
-}
module Stat
where
import Char
data Stat = Assign String Expr
deriving (Show)
data Expr = Ident String
deriving (Show)
type Result = IO
%{
Terminal = EQU | IDENT {String};
Nonterminal = *stat {Stat} | *expr {Expr};
stat {Assign s e} : IDENT {s}, EQU, expr {e};
expr {Ident s} : IDENT {s};
}%
frown ts = fail "syntax error"
data Terminal = IF | THEN | ELSE | EQU | LPAR | RPAR | IDENT String
lexer :: String -> [Terminal]
lexer [] = []
lexer ('(' : cs) = LPAR : lexer cs
lexer (')' : cs) = RPAR : lexer cs
lexer ('=' : cs) = EQU : lexer cs
lexer (c : cs)
| isAlpha c = let (n, cs') = span isAlphaNum cs
in (case (c : n) of
"if" -> IF
"then" -> THEN
"else" -> ELSE
s -> IDENT s) : lexer cs'
| otherwise = lexer cs
|
