id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5936	Support for data families in generics	reinerp	dreixel	"Currently {{{DeriveGeneric}}} doesn't handle data families correctly, so that the following module

{{{
{-# LANGUAGE TypeFamilies, DeriveGeneric #-}
module FamilyDerive where
import GHC.Generics

data family F a
data instance F Int = FInt deriving Generic
}}}

fails to compile, with error

{{{
/tmp/FamilyDerive.hs:6:15:
    Couldn't match type `Rep (F Int)' with `M1 t0 t1 (M1 t2 t3 U1)'
    The type variables `t0', `t1', `t2', `t3' are ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Expected type: Rep (F Int) x
      Actual type: M1 t0 t1 (M1 t2 t3 U1) x
    In the pattern: M1 (M1 U1)
    In an equation for `to': to (M1 (M1 U1)) = FInt
    In the instance declaration for `Generic (F Int)'
}}}

It turns out to require only small changes to GHC to handle data families. I've attached patches.
"	feature request	closed	normal	7.6.1	Compiler	7.4.1	fixed		patrick@… bgamari@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown	generics/GEq1, generics/GenCanDoRep0