/usr/share/doc/frown-doc/examples/other/VarExpr.g is in frown-doc 0.6.1-14.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 | {-
Illustrates the use of patterns with embedded character literals.
frown VarExpr.g
-}
module VarExpr
where
import Char
data AddOp = Plus | Minus
deriving (Show)
data MulOp = Times | Divide
deriving (Show)
data Expr = Add Expr AddOp Expr
| Mul Expr MulOp Expr
| Id String
deriving (Show)
type Result = []
%{
Terminal = IDENT {String} | ADDOP {AddOp} | MULOP {MulOp} | Special '(' | Special ')';
Nonterminal = expr {Expr} | term {Expr} | factor {Expr};
expr {Add e1 op e2} : expr {e1}, ADDOP {op}, term {e2};
{e} | term {e};
term {Mul e1 op e2} : term {e1}, MULOP {op}, factor {e2};
{e} | factor {e};
factor {e} : Special '(', expr {e}, Special ')';
{Id s} | IDENT {s};
}%
frown _ = fail "syntax error"
data Terminal = IDENT String | ADDOP AddOp | MULOP MulOp | Special Char
lexer :: String -> [Terminal]
lexer [] = []
lexer ('+' : cs) = ADDOP Plus : lexer cs
lexer ('-' : cs) = ADDOP Minus : lexer cs
lexer ('*' : cs) = MULOP Times : lexer cs
lexer ('/' : cs) = MULOP Divide : lexer cs
lexer ('(' : cs) = Special '(' : lexer cs
lexer (')' : cs) = Special ')' : lexer cs
lexer (c : cs)
| isAlpha c = let (n, cs') = span isAlphaNum cs
in IDENT (c : n) : lexer cs'
| otherwise = lexer cs
|