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

David Sabel sabel at ki.informatik.uni-frankfurt.de
Mon Jul 10 12:06:10 EDT 2006


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


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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