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April 29, 2012 23:40
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Happy example: attempt to add error handling 2
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-- Calc.y | |
-- Based on http://www.haskell.org/happy/doc/html/sec-using.html | |
{ | |
module Main (main) where | |
import Data.Char | |
} | |
%name calc | |
%tokentype { Token } | |
%error { parseError } | |
%monad { P } { thenP } { returnP } | |
%token | |
let { TokenLet } | |
in { TokenIn } | |
int { TokenInt $$ } | |
var { TokenVar $$ } | |
'=' { TokenEq } | |
'+' { TokenPlus } | |
'-' { TokenMinus } | |
'*' { TokenTimes } | |
'/' { TokenDiv } | |
'(' { TokenOB } | |
')' { TokenCB } | |
%% | |
Exp :: { Int } | |
: let var '=' Exp in Exp {% undefined } | |
{- OOPS: $6 has type Int instead of P Int, | |
meaning that we cannot evaluate it in an extended environment. -} | |
| Exp1 { $1 } | |
Exp1 :: { Int } | |
: Exp1 '+' Term { $1 + $3 } | |
| Exp1 '-' Term { $1 - $3 } | |
| Term { $1 } | |
Term :: { Int } | |
: Term '*' Factor { $1 * $3 } | |
| Term '/' Factor { $1 `div` $3 } | |
| Factor { $1 } | |
Factor :: { Int } | |
: int { $1 } | |
| var {% \p -> case lookup $1 p of | |
Nothing -> failE "no var" | |
Just i -> returnE i } | |
| '(' Exp ')' { $2 } | |
{ | |
type P a = [(String, Int)] -> E a | |
thenP :: P a -> (a -> P b) -> P b | |
m `thenP` k = \r -> | |
m r `thenE` flip k r | |
returnP :: a -> P a | |
returnP = const . returnE | |
failP :: String -> P a | |
failP = const . failE | |
data E a = Ok a | Failed String | |
thenE :: E a -> (a -> E b) -> E b | |
m `thenE` k = | |
case m of | |
Ok a -> k a | |
Failed e -> Failed e | |
returnE :: a -> E a | |
returnE a = Ok a | |
failE :: String -> E a | |
failE err = Failed err | |
parseError :: [Token] -> P a | |
parseError _ = failP "Parse error" | |
data Token | |
= TokenLet | |
| TokenIn | |
| TokenInt Int | |
| TokenVar String | |
| TokenEq | |
| TokenPlus | |
| TokenMinus | |
| TokenTimes | |
| TokenDiv | |
| TokenOB | |
| TokenCB | |
deriving Show | |
lexer :: String -> [Token] | |
lexer [] = [] | |
lexer (c:cs) | |
| isSpace c = lexer cs | |
| isAlpha c = lexVar (c:cs) | |
| isDigit c = lexNum (c:cs) | |
lexer ('=':cs) = TokenEq : lexer cs | |
lexer ('+':cs) = TokenPlus : lexer cs | |
lexer ('-':cs) = TokenMinus : lexer cs | |
lexer ('*':cs) = TokenTimes : lexer cs | |
lexer ('/':cs) = TokenDiv : lexer cs | |
lexer ('(':cs) = TokenOB : lexer cs | |
lexer (')':cs) = TokenCB : lexer cs | |
lexNum cs = TokenInt (read num) : lexer rest | |
where (num,rest) = span isDigit cs | |
lexVar cs = | |
case span isAlpha cs of | |
("let", rest) -> TokenLet : lexer rest | |
("in", rest) -> TokenIn : lexer rest | |
(var, rest) -> TokenVar var : lexer rest | |
main :: IO () | |
main = do | |
cs <- getContents | |
case calc (lexer cs) [] of | |
Ok res -> print res | |
Failed err -> putStrLn err | |
} |
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