GHC and Haskell 98

[Jacques Carette]

On 17/06/2011 10:47 AM, Simon Peyton-Jones wrote:
> But Plan A is simpler. And by breaking packages it will encourage [force] libraries that use a mixture of H98 and more modern modules to move towards the more modern story.

I favour Plan A.

Reasoning:
For many years of my previous professional life, I had to live with 
extreme backwards compatibility in the code base.  This introduced a 
huge amount of inertia, to the point where past design mistakes became 
entrenched as 'features'.  Forward progress became glacial.  Very bad.

Your plan A on the other hand strikes a really good balance: the only 
libraries affected are those which mix H98 and more modern modules.  
This means that the authors are already beyond Haskell 98, and realize 
that there is real value to go beyond that.  So they should be 
reasonably amenable to continue to move forward.

On the other hand, those who were very careful to stick to pure Haskell 
98 (for whatever reason), have a very clear path for their code to 
continue to stay functional.  They get all the backwards compatibility 
they desired when they chose to stick to pure Haskell 98.  Plan B is 
actually more fragile in that respect, in that if they forget to be 
really really explicit about their code being pure Haskell 98, the 
resulting compilation errors do not make it obvious that that is 
actually the problem.  This will in fact only get worse as time goes by.

Jacques