/usr/share/doc/frown-doc/examples/other/MCalc.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 | {-
This example demonstrates monadic semantic actions.
frown MCalc.g
Try
expr (lexer "6*8+5")
-}
module MCalc
where
import Char
type Op = Int -> Int -> Int
type Result = IO
%{
Terminal = NAT {Int} | ADDOP {Op} | MULOP {Op} | LPAR | RPAR | *EOF;
Nonterminal = expr {Int} | term {Int} | factor {Int};
expr {%trace (v1 `op` v2)} : expr {v1}, ADDOP {op}, term {v2};
{e} | term {e};
term {%trace (v1 `op` v2)} : term {v1}, MULOP {op}, factor {v2};
{e} | factor {e};
factor {e} : LPAR, expr {e}, RPAR;
{n} | NAT {n};
}%
frown ts = fail "syntax error"
trace v = print v >> return v
data Terminal = NAT Int | ADDOP Op | MULOP Op | LPAR | RPAR | EOF
lexer :: String -> [Terminal]
lexer [] = [EOF]
lexer ('+' : cs) = ADDOP (+) : lexer cs
lexer ('-' : cs) = ADDOP (-) : lexer cs
lexer ('*' : cs) = MULOP (*) : lexer cs
lexer ('/' : cs) = MULOP div : lexer cs
lexer ('(' : cs) = LPAR : lexer cs
lexer (')' : cs) = RPAR : lexer cs
lexer (c : cs)
| isDigit c = let (n, cs') = span isDigit cs
in NAT (read (c : n)) : lexer cs'
| otherwise = lexer cs
|