Cannot deduce X error with X provided in TypeFamilies
The following code
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
module TorXakis.Test
( -- * Referable
Referable(..)
)
where
import Data.Hashable
import qualified Data.HashMap as HashMap
-- | A referable class
class Referable a where
type Ref a
toRef :: a -> Ref a
-- | Map of Referable objects.
data (Referable a, Ord (Ref a), Hashable (Ref a)) =>
RefMap a = RefMap { -- | the HashMap
toHashMap :: HashMap.Map (Ref a) a
} deriving (Eq, Ord, Show, Read)
gives the following errors
C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:43: error:
* Could not deduce (Ord (Ref a))
arising from the 'deriving' clause of a data type declaration
from the context: (Eq a, Referable a)
bound by the deriving clause for `Eq (RefMap a)'
at src\TorXakis\Test.hs:20:43-44
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
* When deriving the instance for (Eq (RefMap a))
|
20 | } deriving (Eq, Ord, Show, Read)
| ^^
C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:47: error:
* Could not deduce (Hashable (Ref a))
arising from the 'deriving' clause of a data type declaration
from the context: (Ord a, Referable a)
bound by the deriving clause for `Ord (RefMap a)'
at src\TorXakis\Test.hs:20:47-49
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
* When deriving the instance for (Ord (RefMap a))
|
20 | } deriving (Eq, Ord, Show, Read)
| ^^^
C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:52: error:
* Could not deduce (Ord (Ref a))
arising from the 'deriving' clause of a data type declaration
from the context: (Show a, Referable a)
bound by the deriving clause for `Show (RefMap a)'
at src\TorXakis\Test.hs:20:52-55
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
* When deriving the instance for (Show (RefMap a))
|
20 | } deriving (Eq, Ord, Show, Read)
| ^^^^
C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:58: error:
* Could not deduce (Ord (Ref a))
arising from the 'deriving' clause of a data type declaration
from the context: (Read a, Referable a)
bound by the deriving clause for `Read (RefMap a)'
at src\TorXakis\Test.hs:20:58-61
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
* When deriving the instance for (Read (RefMap a))
|
20 | } deriving (Eq, Ord, Show, Read)
| ^^^^
-- While building package txs-basics-0.1.0.0 using:
C:\sr\setup-exe-cache\x86_64-windows-integersimple\Cabal-simple_Z6RU0evB_2.0.1.0_ghc-8.2.2.exe --builddir=.stack-w
ork\dist\67675594 build lib:txs-basics --ghc-options " -ddump-hi -ddump-to-file"
Process exited with code: ExitFailure 1
yet the requirements such as Ord (Ref a)
are clearly given to the data definition!
possibly related to #16319 (closed)
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |