instance overlap in 6.6 candidate

[Serge D. Mechveliani]

On Mon, Sep 04, 2006 at 03:39:06PM +0100, Simon Peyton-Jones wrote:
> | ------------------------------------------------
> | data Equation = ...
> | instance Show Equation where ...
> | 
> | instance Show [Equation]
> |   where
> |   showsPrec _ eqs =  <certain program which prints a list of equation
> |                       in a `nicer' way than by the default list
> printing
> |                      >
> | ------------------------------------------------
> | 
> | This gives the user possibility to redefine a certain part of the
> | library instance.
> | Is not  ghc-6.4.1  better at this point?
> 

> Perhaps.  That's what I'd like feedback about. 


Let me exlain a bit more the reason.  

show [1, 2, 3] =  "[1,2,3]"   -- all right, let it be.

Now, for the list  eqs :: [Equation],  the GHC instance prints it, for 
example, as
-------------------
[[21]  n+0 = 0   (Parents [[1],[2]]),[22]  n+(s n) = s (n+m)   (Parents [[33],
[44]]),[23]  n*0 = 0   (Parents [[],[]])]
-------------------

And I would like it to be at least
--------------------
[[21]  n+0 = 0   (Parents [[1],[2]]),
 [22]  n+(s n) = s (n+m)   (Parents [[33],[44]]),
 [23]  n*0 = 0   (Parents [[],[]])
],
--------------------

which is easier to read. I decided that to overlap with the ghc instance 
is the simplest and best solution.
I thank  Ian Lynagh <igloo at earth.li>  who suggests

L> instance Show Equation where
L>    showsPrec _ eq = ...
L>    showList eqs = <certain program which ...>

Indeed. I even wrote a  showsList  function, without knowing of the  
showsList  class method!  
But as Ian writes,
L> This doesn't address the general issue

So far, I tried this overlap only for  Show List and Show Pair,  
but probably, users and myself would like to partially redefine some other 
library instances. 


Simon Peyton-Jones writes

> (Incidentally, 6.5 has been like this for about a year without
> complaints. 

I am sorry for this, I am slower than GHC. I have been with 6.4.1 last 
1-2 years, and ignored the further version matters. Finally, a week ago 
decided to look into what is coming with 6.6.

> Also I made the change because people wanted to be able to
> *use* overlapping instances (defined in a library) without having to
> give the -fallow-overlapping-instances flag.)

The difference for me is to have a little bit less-or-more comfort.

DoCon-2.09 has ported from  6.4.1  to  6.6-candidate  only by removing 
the `data' name from `dependencies', and it looks like working all right.
In Dumatel-1.06, it reports the error of this library overlap for `Show'.
For exampl, I can define the  showsList  method, as Ian suggests.

Generally, I suspect that ghc-6.4.1 is better at this point of library 
overlaps, but I am not sure.
Anyway, the GHC developers have now more of feedback, they know better 
and can decide. 

Thanks,

-----------------
Serge Mechveliani
mechvel at botik.ru