Very useful to get a gadt back to monotype without an existential, which would mean to use classes for future uses of it with its load of object oriented thinking.
Thanks for sharing. paolino 2012/6/4 Ryan Ingram <ryani.s...@gmail.com> > Another option is to reify the type so that you can get it back somehow. > Here's a few diffs to your file (I've attached the full code): > > A new type: > data Typed f where > TDouble :: f Double -> Typed f > TBool :: f Bool -> Typed f > > runT :: (f Double -> a) -> (f Bool -> a) -> Typed f -> a > runT k _ (TDouble x) = k x > runT _ k (TBool x) = k x > > New version of pExpr that can parse both expression types, by tagging with > the type > > -- pExpr = pArit <|> pBool <|> pEqual > pExpr = (TDouble <$> pArit) <|> (TBool <$> pBool) <|> (TDouble <$> > pEqual) > > and now main: > main = do line <- getLine > case parse pExpr "" line of > Left msg -> putStrLn (show msg) > Right e -> putStrLn (runT (show . eval) (show . eval) e) > > What I'm doing here is reifying the possible types of top level > expressions and then providing a handler in main which works on all > possible types. There are other ways to do this (embed any expression in > an existential, for example), but this makes it really clear what is going > on, and shows the way forward for parsing a larger typed language. > > -- ryan > > On Wed, May 2, 2012 at 6:08 AM, <j.romi...@gmail.com> wrote: > >> On Wed, May 02, 2012 at 03:02:46PM +0300, Roman Cheplyaka wrote: >> > * j.romi...@gmail.com <j.romi...@gmail.com> [2012-05-02 08:03:45-0300] >> [...] >> > The alternatives given to <|> must be of the same type. In your case, >> > one is Expr Double and one is Expr Bool. >> > >> > Inclusion of pBool in pFactor is probably a mistake — unless you're >> > going to multiply booleans. >> >> You are right in the sense that I cannot mix Expr Bool and Expr Double >> in a (O op l r) expression. >> >> But the parser should be able to parse any form of expressions. So I >> rewrite my program to take this into account. >> >> The new versions still does not compile: >> >> Expr.hs:27:23: >> Couldn't match expected type `Double' with actual type `Bool' >> Expected type: ParsecT >> String () Data.Functor.Identity.Identity (Expr Double) >> Actual type: ParsecT >> String () Data.Functor.Identity.Identity (Expr Bool) >> In the first argument of `(<|>)', namely `pBool' >> In the second argument of `(<|>)', namely `pBool <|> pEqual' >> >> Romildo >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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