#7487: Deriving Generic1 for a type containing Either
-----------------------------+----------------------------------------------
Reporter: spl | Owner: dreixel
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Blockedby:
Blocking: | Related:
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With the following code:
{{{
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data T a = C (Either Int a) deriving Generic1
}}}
I get the error message:
{{{
Main.hs:3:38:
Can't make a derived instance of `Generic1 T':
must not apply type constructors that cannot be represented with
`Rep1'
(such as `Either') to arguments that involve the last type parameter
In the data declaration for `T'
}}}
But when I look at the instances of `Generic1`, I see the following
(summarized):
{{{
*Main> :i Generic1
...
instance Generic1 (Either a) -- Defined in `GHC.Generics'
...
}}}
So, it seems that `Either a` can be represented with `Rep1`. Consequently,
the error message is a bit confusing.
Similarly, I can define my own version of `Either` -- e.g. `data U a b = A
a | B b deriving Generic1` -- replace `Either` with `U` and still get the
message. I also get this problem using `U` as `data U a b = U a b deriving
Generic1` but not with `T` as `data T a = C (Int, a) deriving Generic1`.
Lastly, I try this:
{{{
data T a = C (Either Int (T a)) deriving Generic1
}}}
but the error is even stranger here:
{{{
Main.hs:3:38:
Can't make a derived instance of `Generic1 T':
must not apply type constructors that cannot be represented with
`Rep1'
(such as `T') to arguments that involve the last type parameter
In the data declaration for `T'
}}}
What's the general problem with these examples?
* Can the error message be improved?
* Can the deriving mechanism be extended for these types?
I'm assuming I can write my own `Generic1` instance, though I haven't
actually tried.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7487>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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