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Allow derive newtype instance for newtypes that use type synonyms #2481
Description
My minimal reproduction is
type MyString = String
newtype WrappedString = WrappedString MyString
derive newtype instance semigroupWrappedString :: Semigroup WrappedString
which produces the error
No type class instance was found for
Data.Semigroup.Semigroup MyString
while checking that expression #dict Semigroup
has type { append :: WrappedString -> WrappedString -> WrappedString
}
in value declaration semigroupWrappedString
even though
newtype WrappedString = WrappedString String
derive newtype instance semigroupWrappedString :: Semigroup WrappedString
compiles without error.
(Originally posted incorrectly to #2416, but this is a different bug.)
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paf31 commentedon Dec 14, 2016
I thought this was the same bug as #2416. How is it different?
joshuahhh commentedon Dec 14, 2016
@paf31: #2416 is about deriving instances of
Newtypefor newtypes which use type synonyms, like#2481 is about deriving newtype instances (via
derive newtype instance) of other typeclasses for newtypes which use type synonyms, likeI don't actually know what's going on behind the scenes here, so these may be the same problem at a deeper level.
But #2416 is, literally speaking, just about a failure to derive instances of capital-N
Newtype, so I thought this might be something new.paf31 commentedon Dec 14, 2016
I thought it was the other way around. Either way, the other issue is #1443, and I think this duplicates one or the other.
garyb commentedon Dec 14, 2016
The other issue is not #1443, as we discussed before 😛, although once the other issue is fixed, 1443 will probably become a problem too. This one is definitely closer to #1443, and maybe will be fixed by the same thing.