Akio Thanks that's amazing! I wonder if it'd be worth adding it, with commentary, (and attaching the files) as a Trac ticket? I know its already fixed, but it's great to be able to say "to see an example of why this is vital, see Trac #8324", whereas email archives are more ephemeral somehow. Simon From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Akio Takano Sent: 23 August 2013 11:35 To: glasgow-haskell-users@haskell.org; Richard Eisenberg Subject: Fwd: New restrictions to open type families (I forgot to send to the list, trying again) Thank you very much for your detailed explanation! On Fri, Aug 23, 2013 at 1:24 PM, Richard Eisenberg mailto:eir@cis.upenn.edu> wrote: This is a good question. Happily, there are at least two decent answers. 1) We're not sure that this problem cannot cause a segfault... it's just that we've been unable to produce one when trying. Perhaps we haven't tried hard enough. I successfully constructed such an example. https://github.com/takano-akio/type-family-overlap - Akio

Yes -- wow. I had tried similar techniques, but couldn't get it to go through in the end. I think using the datatype was the key. Cool! I will update the wiki as per Simon's suggestions. On Aug 23, 2013, at 6:34 AM, Akio Takano wrote:

(I forgot to send to the list, trying again)

Thank you very much for your detailed explanation!

On Fri, Aug 23, 2013 at 1:24 PM, Richard Eisenberg wrote: This is a good question. Happily, there are at least two decent answers.

1) We're not sure that this problem cannot cause a segfault… it's just that we've been unable to produce one when trying. Perhaps we haven't tried hard enough.

I successfully constructed such an example.

https://github.com/takano-akio/type-family-overlap

- Akio