[Haskell-cafe] Prolog-style patterns

Daniel Trstenjak daniel.trstenjak at gmail.com
Tue Apr 9 15:07:20 CEST 2013

Hi Jan,

On Tue, Apr 09, 2013 at 01:32:33PM +0200, Jan Stolarek wrote:
> Thanks for pointing me to earlier discussions on this subject - they are enlightening :) One 
> particular argument "against" seems to be very convincing to me:
> "From a language design point of view, it should be noted that turning
> non-left linear patterns into ones with == guards elevates the class Eq
> to built-in status - but the compiler has no semantic control over it."

Yes, I can see the point, but in the case of Haskell with its ability to
automatically derive the Eq instance, there's some kind of semantic
control, and if an user writes a nonsense Eq instance, than he just
really asks for some hurting ;). 

> Regarding the possibility of making accidental mistakes during refactoring etc. This could be 
> implemented as a language extension, requiring explicit LANGUAGE pragma. So people using it would 
> know they have semantics slightly changed and need to be aware that there is a possibility of 
> making these kind of mistakes.

I'm a bit torn between all these GHC extensions. If your code doesn't
compile, did you really made a mistake or just forgot to include a GHC
extension ...

But I also have the feeling, that the extension perhaps might not be
worth it, because the difference between

foo x x = ...


foo x y | x == y = ...

is just IMHO too small.


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