Hi, you can make every function being an instance of class Show, this works for me: instance Show (a -> b) where show _ = "FUNCTION" data Element = Int Int | Float Float | Func (Machine -> Machine) deriving Show David Johan Grönqvist wrote:

I am a haskell-beginner and I wish to write a Forth-like interpreter. (Only for practice, no usefulness.)

I would like use a list (as stack) that can contain several kinds of values.

data Element = Int Int | Float Float | Func : Machine -> Machine | ...

Now I would like to have this type be an instance of the class Show, so that I can see what the stack contains in ghci.

"deriving Show" is impossible as Func is not instance of Show. Can I make it instance of Show? I just want to define something like

show (Func _) = "Function, cannot show"

and I am not interested in actually displaying any information about the function, but I am interested in getting information about elements of other kinds.

So far I have just guessed, but failed to produce anything that does not give an error stating that my function is not of the form (T a b c) where T is not an alias and a b c are simple type variables (or so).

Any help appreciated!

Johan

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johan.gronqvist:

I am a haskell-beginner and I wish to write a Forth-like interpreter. (Only for practice, no usefulness.)

I would like use a list (as stack) that can contain several kinds of values.

data Element = Int Int | Float Float | Func : Machine -> Machine | ...

Now I would like to have this type be an instance of the class Show, so that I can see what the stack contains in ghci.

Here's an interesting, I think, show for functions that we use in lambdabot's Haskell interpreter environment: module ShowQ where import Language.Haskell.TH import System.IO.Unsafe import Data.Dynamic instance (Typeable a, Typeable b) => Show (a -> b) where show e = '<' : (show . typeOf) e ++ ">" instance Ppr a => Show (Q a) where show e = unsafePerformIO $ runQ e >>= return . pprint which generates results like: dons:: > toUpper lambdabot:: Char> dons:: > \x -> x+1::Int lambdabot:: Int> dons:: > map lambdabot:: Add a type signature Note that also the standard libraries come with Text.Show.Functions -- Don

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"Johan" == Johan Grönqvist writes:

Johan> I am a haskell-beginner and I wish to write a Forth-like Johan> interpreter. (Only for practice, no usefulness.) Johan> I would like use a list (as stack) that can contain several Johan> kinds of values. Johan> data Element = Int Int | Float Float | Func : Machine -> Johan> Machine | ... Johan> Now I would like to have this type be an instance of the class Johan> Show, so that I can see what the stack contains in ghci. Johan> "deriving Show" is impossible as Func is not instance of Johan> Show. Can I make it instance of Show? I just want to define Johan> something like Johan> show (Func _) = "Function, cannot show" err, why not just write instance Show Element where ... show (Func _) = "Function, cannot show" E.g: data Foo = Bar | Foo (Int -> Int -> Int) instance Show Foo where show Bar = "Bar" show (Foo _) = "Func!" main = do putStrLn $ show Bar putStrLn $ show $ Foo (+) -- WBR, Max Vasin. NP: Nothing playing right now