[Haskell-cafe] Re: Defining show for a function type.

11 Jul 2006

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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.

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