We could do that, but personally I don't think it's worth it. I'm ccing ghc-devs in case anyone has a strong opinion. Simon | -----Original Message----- | From: Richard Eisenberg [mailto:eir@cis.upenn.edu] | Sent: 27 August 2013 21:45 | To: Simon Peyton-Jones; Dimitrios Vytiniotis | Cc: Stephanie Weirich | Subject: GHC TypeFamilies extension | | Hi Simon, Dimitrios, | | Among the various responses I've gotten to closed type families was a | complaint (made by a fellow Penn student, Daniel Wagner) about | extensions that change their meaning over time. Here is why: If a | package requires the use of a certain extension, it can declare the | extension in the cabal file, so that end users get a nice error message | if their GHC is out of date. However, if a package uses closed type | families, no such declaration is possible; an old GHC will just die with | a parse error. There are ways to insist on a certain version of GHC or a | version of the base library, but that doesn't seem great, either. | | On the other hand, I'm loathe to add yet another extension | ClosedTypeFamilies, forcing yet another pragma at the top of | programmers' files. | | Instead, we came up with this intermediate solution: | - Introduce two new extensions, OpenTypeFamilies and ClosedTypeFamilies | - TypeFamilies would imply both of these | - Appropriate checks would be made and error messages improved | | Now, users can still use TypeFamilies to get both flavors of type | families, but a package maintainer can specify ClosedTypeFamilies in a | cabal file to control which versions of GHC are usable. | | I'm still not 100% convinced this is the right idea, but there is enough | sense in it to ask. | | What do you think? If you all think this is a good enough idea, I'll | forward to glasgow-haskell-users for a wider discussion. | | Thanks, | Richard