Question

I want to use Debug.Trace.trace to print something which I know is a Show. Just like I would do in Python etc.

One solution is to add “Show a =>” to the signature of the function where I want to put the trace, and to any function calling it, etc.

But it would had been much nicer if I could use some debugShow function which calls show if the value has one, otherwise returns "--no show--" or something.

Here’s my failed attempt to define a DebugShow (GHC rejects "Duplicate instance declarations"):

{-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances #-}

class DebugShow a where
  debugShow :: a -> String

instance Show a => DebugShow a where
  debugShow = show

instance DebugShow a where
  debugShow = const "--no show--"

Some kind of “unsafe cast” would also solve my problem.

Any advice?

Note – This is only for debugging purposes. I’m not using this in any finished code.

Answer 1

I’m not absolutely certain of this, but I think this is impossible without adding a class context to the entire call chain between the use site and the call site where each type variable is determined. The reason for this is operational: at least in GHC, each class is implemented by a dictionary. So at the use site we need a Show dictionary for a if one exists at all. But to get this we need it to have been passed down from the site where a was determined, and this requires something to be in the signature of all the intermediate functions.