I think it's pretty good as-is.
* Use an open family (with non-overlapping instances) to get yourself
into part of the match space:
type instance OpenF (Foo b c) = FFoo (Foo b c)
* Use a closed family (with overlap and top-to-bottom matching) to
deal with that part of the space:
type family FFoo a where
FFoo (Foo Int c) = ...
Doing this was a HUGE improvement, allowing us to cleanly split the
issues of top-to-bottom matching from those of non-overlapping open
families.
| It also BTW cuts us off from using Closed Families as Associated types
| separated into their Class instances.
I don't understand this comment. Can you give an example that the current setup does not handle?
Simon
| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces@haskell.org] On Behalf Of AntC
| Sent: 04 June 2015 01:54
| To: glasgow-haskell-users@haskell.org
| Subject: Re: Closed Type Families: separate instance groups?
|
| > Richard Eisenberg writes:
| >
| > You can always define a helper closed type family and have an open
| > type family instance just call a closed type family.
|
| Thank you Richard, you mean like:
|
| type family OpenF a
|
| ...
|
| type instance OpenF (Foo b c) = FFoo (Foo b c)
| type family FFoo a where
| FFoo (Foo Int c) = ...
| ...
|
| OK. (Seems rather verbose.)
|
| >
| > Having closed type families, as opposed to branched instances, just
| > seemed like a cleaner way to package the new functionality.
| > There really wasn't much to it other than aesthetics, if I recall
| the
| > conversations correctly.
|
| I recall the conversation quite well.
| (In fact I think it was me who suggested type family ... where ... ) I
| think it was less to do with aesthetics, and more to do with reducing
| verbosity in a common use case.
|
| It somehow doesn't seem as clean as old-fashioned overlapping
| instances.
| (I agree it does seem cleaner than overlaps with FunDeps.)
|
| It also BTW cuts us off from using Closed Families as Associated types
| separated into their Class instances.
| I think there's two use cases going on:
| - one where we want to see all the instances together
| that fits well to type family ... where ...
| - t'other where we want everything to do with a type constructor
| together
| that fits better with the separate instances
|
| AntC
|
|
|
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