On Thu, Apr 25, 2013 at 6:38 AM, harry <volderm...@hotmail.com> wrote:
> If I understand correctly, the problem with datatype contexts is that if we > have e.g. > data Eq a => Foo a = Foo a > the constraint Eq a is thrown away after a Foo is constructed, and any > method using Foos must repeat Eq a in its type signature. > > Why were these contexts removed from the language, instead of "fixing" > them? > As I understand it, it's because fixing them involves passing around a dictionary along with the data, and you can't do that with a standard declaration (it amounts to an extra chunk of data that's only *sometimes* wanted, and that "sometimes" complicates things). GADTs already have to pass around extra data in order to support their constructors and destructors; and, being new and not part of the standard, they don't have backward compatibility or standards compatibility issues, so they can get away with including the extra dictionary without breaking existing programs. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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