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The standard example: arithmetic expressions (see dragon book p222).
Missing mutiplication signs are inserted.
frown --expected IExpr.lg
Try
expr (lexer "a+b*c+d") :: [Expr]
expr (lexer "a+b c+d") :: [Expr]
> module IExpr
> where
> import Char
>
> data Expr = Id String
> | Add Expr Expr
> | Mul Expr Expr
> deriving (Show)
>
> type Result = []
>
> %{
>
> Terminal = Ident {String} as "<identifier>"
> | Addop as "+"
> | Mulop as "*"
> | LPar as "("
> | RPar as ")";
>
>*expr {Expr};
> expr {Add e1 e2} : expr {e1}, "+", term {e2};
>-- {Add e1 e2} | expr {e1}, insert "+", term {e2};
> {e} | term {e};
>
> term {Expr};
> term {Mul e1 e2} : term {e1}, "*", factor {e2};
> {Mul e1 e2} | term {e1}, insert "*", factor {e2};
> {e} | factor {e};
>
> factor {Expr};
> factor {e} : "(", expr {e}, ")";
> {Id s} | Ident {s};
>
> }%
>
> frown la ts = fail "syntax error"
>
> data Terminal = Ident String
> | Addop
> | Mulop
> | LPar
> | RPar
> deriving (Show)
>
> lexer :: String -> [Terminal]
> lexer [] = []
> lexer ('+' : cs) = Addop : lexer cs
> lexer ('*' : cs) = Mulop : lexer cs
> lexer ('(' : cs) = LPar : lexer cs
> lexer (')' : cs) = RPar : lexer cs
> lexer (c : cs)
> | isAlpha c = let (n, cs') = span isAlphaNum cs
> in Ident (c : n) : lexer cs'
> | otherwise = lexer cs
|
