The dreaded M-R

Simon Marlow simonmar at
Tue Jan 31 05:17:57 EST 2006

On 30 January 2006 21:49, Andrew Pimlott wrote:

> On Mon, Jan 30, 2006 at 04:45:56PM -0000, Simon Marlow wrote:
>> Given the new evidence that it's actually rather hard to demonstrate
>> any performance loss in the absence of the M-R with GHC, I'm
>> attracted to the option of removing it in favour of a warning.
> I caution against the hope that warnings will contribute to the
> solution, whichever side you're on.  This is a general argument: 
> Either the warning is on by default or off.  If off, it does no harm,
> but doesn't help much either.  If on, it either triggers only on code
> that is almost certainly wrong (or easily disambiguated), or it
> sometimes triggers on perfectly good code.  In the former case, it
> would be better to make it illegal (or require the disambiguation). 
> In the latter, nobody likes disabling warnings, so they'll grumble
> and change the code instead.
> In the present case, people aren't (only) opposing the M-R out of
> principle, but because they actually use overloaded variable
> definitions and (at least sometimes) want to leave off the signature.
> So I don't see how one could claim, as on the wiki, the warning
> "wouldn't happen much".  I suspect it would happen, and annoy people,
> and defeat the reason that people want to remove the M-R.

The assertion that it "wouldn't happen much" is based on the observation
earlier in this thread that it was actually difficult to write some code
that illustrated the problem.

Nevertheless, I didn't mean to imply that the language would mandate a
warning, I'll change the wiki to make this more clear.


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