Deriviable type classes

Sean Leather leather at
Wed Feb 9 15:10:58 CET 2011

2011/2/9 Simon Peyton-Jones:

> Friends
> Just a heads-up.  Pedro is working on implementing "Generic Defaults", as
> described in his Haskell Symposium 2010 paper
> It will replace (and improve on) the "Derivable type classes" stuff in GHC
> at the moment, which was originally presented in a paper of that title that
> Ralf and I wrote in the 2000 Haskell workshop
> The "Derivable type class" extension is barely used, I believe, and isn't
> even documented in the manual.  So I propose to switch from one to the
> other, rather than to try to support both.  This change will happen in GHC
> 7.2 or 7.4, depending on when Pedro is done.
> Please yell if you are a secret user of derivable type classes, so this
> change would discombobulate you.

The only time I came across a use of it was in James Cheney's FreshLib [1].
I don't know if the latest version still uses it, though. There was also
this bug [2] that prevented FreshLib from being compiled, but that is
apparently fixed now.


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