#13262: Allow type synonym family application in instance head if it has no free variables -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: 12680 Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Artyom.Kazak):
Oh all right. We can wait till someone else pipes in and requests this :)
That would be me! My usecase is generating anonymous records and then defining SafeCopy migrations for them (see http://hackage.haskell.org/package/safecopy-0.9.4.1/docs/Data- SafeCopy.html#t:Migrate). Essentially this: {{{ type Foo_v1 = ... type Foo_v2 = AddField "name" Text Foo_v1 type Foo_v3 = RemoveField "age" Foo_v2 }}} All of that is implemented via type families. The problem is that now I can't have instances for these types (e.g. `instance Migrate Foo_v3`). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13262#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler