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

Johan Grönqvist johan.gronqvist at gmail.com
Mon Jul 10 11:44:15 EDT 2006

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!


More information about the Haskell-Cafe mailing list