Thanks. So GADT provide us to generic type to include all types, is it right? ex: data Parser tok a where Zero :: Parser tok () One :: Parser tok () Check :: (tok -> Bool) -> Parser tok tok Satisfy :: ([tok] -> Bool) -> Parser tok [tok] Push :: tok -> Parser tok a -> Parser tok a Plus :: Parser tok a -> Parser tok b -> Parser tok (Either a b) Times :: Parser tok a -> Parser tok b -> Parser tok (a,b) Star :: Parser tok a -> Parser tok [a] otherwise we need to write something like this (separate the type and functions): newtype Parser tok a zero::Parser tok () one :: Parser tok () ... On Nov 27, 2007 2:16 PM, Luke Palmer wrote:

On Nov 27, 2007 12:57 PM, Yu-Teh Shen wrote:

...

i have seen the documents in http://www.haskell.org/haskellwiki/Generalised_algebraic_datatype but i can not run the following code on ghci

ex:

data Term x where K :: Term (a -> b -> a) S :: Term ((a -> b -> c) -> (a -> b) -> a -> c) Const :: a -> Term a (:@) :: Term (a -> b) -> (Term a) -> Term b infixl 6 :@

could any tell me how to run the code?

Put it in a file (eg ski.hs), and run:

% ghci -fglasgow-exts ski.hs

You cannot enter it directly into ghci; you can't define new data types interactively.

You can also put the line

{-# OPTIONS_GHC -fglasgow-exts #-}

At the top, to turn on glasgow extensions whenever GHC compiles this file.

Luke

On Tue, Nov 27, 2007 at 07:11:27PM +0000, Andrew Coppin wrote:

I was under the impression that it's better to use the LANGUAGE pragma rather than the catch-all Glasgow-exts option. However, I can't actually find a language option for GADTs... somebody care to clarify?

In GHC 6.8.1 it's "GADTs". "ghc --supported-languages" lists them all. Thanks Ian