Mixing
Deriving Via With Standalone Deriving
I'm not really sure what I'm doing wrong here:
data Vector2D u = Vector2D {
_x :: u,
_y :: u
} deriving stock (Show, Eq, Functor, Foldable, Traversable)
{-# INLINE addVector2 #-}
addVector2 :: (Additive a) => Vector2D a -> Vector2D a -> Vector2D a
addVector2 (Vector2D { _x = x1, _y = y1 }) (Vector2D { _x = x2, _y = y2 }) =
Vector2D { _x = x1 + x2, _y = y1 + y2 }
instance (Additive a) => Additive (Vector2D a) where
(+) = addVector2
newtype Square a = Square {
unpackSquare :: Vector2D a
} deriving stock (Show, Eq)
So far, so normal (Additive is defined in the algebra package but is pretty self-explanatory).
However, now I want to be clever using DerivingVia and StandaloneDeriving and I can't even get the next line to compile
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
But that gets me
* Expected kind `k0 -> * -> Constraint',
but `Additive (Square a)' has kind `Constraint'
* In the stand-alone deriving instance for
`(Additive a) => Additive (Square a) via (Vector2D a)'
Can anyone tell me what I'm doing wrong? I'm running GHC 8.6.2
haskell deriving
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
add a comment |
I'm not really sure what I'm doing wrong here:
data Vector2D u = Vector2D {
_x :: u,
_y :: u
} deriving stock (Show, Eq, Functor, Foldable, Traversable)
{-# INLINE addVector2 #-}
addVector2 :: (Additive a) => Vector2D a -> Vector2D a -> Vector2D a
addVector2 (Vector2D { _x = x1, _y = y1 }) (Vector2D { _x = x2, _y = y2 }) =
Vector2D { _x = x1 + x2, _y = y1 + y2 }
instance (Additive a) => Additive (Vector2D a) where
(+) = addVector2
newtype Square a = Square {
unpackSquare :: Vector2D a
} deriving stock (Show, Eq)
So far, so normal (Additive is defined in the algebra package but is pretty self-explanatory).
However, now I want to be clever using DerivingVia and StandaloneDeriving and I can't even get the next line to compile
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
But that gets me
* Expected kind `k0 -> * -> Constraint',
but `Additive (Square a)' has kind `Constraint'
* In the stand-alone deriving instance for
`(Additive a) => Additive (Square a) via (Vector2D a)'
Can anyone tell me what I'm doing wrong? I'm running GHC 8.6.2
haskell deriving
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
1
For what it is worth, I prefer your syntax withvia ...at the end to the one actually accepted by GHC, whereviacomes earlier.
– chi
Nov 21 '18 at 19:50
add a comment |
I'm not really sure what I'm doing wrong here:
data Vector2D u = Vector2D {
_x :: u,
_y :: u
} deriving stock (Show, Eq, Functor, Foldable, Traversable)
{-# INLINE addVector2 #-}
addVector2 :: (Additive a) => Vector2D a -> Vector2D a -> Vector2D a
addVector2 (Vector2D { _x = x1, _y = y1 }) (Vector2D { _x = x2, _y = y2 }) =
Vector2D { _x = x1 + x2, _y = y1 + y2 }
instance (Additive a) => Additive (Vector2D a) where
(+) = addVector2
newtype Square a = Square {
unpackSquare :: Vector2D a
} deriving stock (Show, Eq)
So far, so normal (Additive is defined in the algebra package but is pretty self-explanatory).
However, now I want to be clever using DerivingVia and StandaloneDeriving and I can't even get the next line to compile
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
But that gets me
* Expected kind `k0 -> * -> Constraint',
but `Additive (Square a)' has kind `Constraint'
* In the stand-alone deriving instance for
`(Additive a) => Additive (Square a) via (Vector2D a)'
Can anyone tell me what I'm doing wrong? I'm running GHC 8.6.2
haskell deriving
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
I'm not really sure what I'm doing wrong here:
data Vector2D u = Vector2D {
_x :: u,
_y :: u
} deriving stock (Show, Eq, Functor, Foldable, Traversable)
{-# INLINE addVector2 #-}
addVector2 :: (Additive a) => Vector2D a -> Vector2D a -> Vector2D a
addVector2 (Vector2D { _x = x1, _y = y1 }) (Vector2D { _x = x2, _y = y2 }) =
Vector2D { _x = x1 + x2, _y = y1 + y2 }
instance (Additive a) => Additive (Vector2D a) where
(+) = addVector2
newtype Square a = Square {
unpackSquare :: Vector2D a
} deriving stock (Show, Eq)
So far, so normal (Additive is defined in the algebra package but is pretty self-explanatory).
However, now I want to be clever using DerivingVia and StandaloneDeriving and I can't even get the next line to compile
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
But that gets me
* Expected kind `k0 -> * -> Constraint',
but `Additive (Square a)' has kind `Constraint'
* In the stand-alone deriving instance for
`(Additive a) => Additive (Square a) via (Vector2D a)'
Can anyone tell me what I'm doing wrong? I'm running GHC 8.6.2
haskell deriving
haskell deriving
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
edited Nov 21 '18 at 10:10
leftaroundabout
79.4k3118233
79.4k3118233
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
asked Nov 20 '18 at 23:21
Julian BirchJulian Birch
1,476923
1,476923
1
For what it is worth, I prefer your syntax withvia ...at the end to the one actually accepted by GHC, whereviacomes earlier.
– chi
Nov 21 '18 at 19:50
add a comment |
1
For what it is worth, I prefer your syntax withvia ...at the end to the one actually accepted by GHC, whereviacomes earlier.
– chi
Nov 21 '18 at 19:50
1
1
For what it is worth, I prefer your syntax with
via ... at the end to the one actually accepted by GHC, where via comes earlier.– chi
Nov 21 '18 at 19:50
For what it is worth, I prefer your syntax with
via ... at the end to the one actually accepted by GHC, where via comes earlier.– chi
Nov 21 '18 at 19:50
add a comment |
1 Answer
1
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It's
deriving via (Vector2D a) instance (Additive a) => Additive (Square a)
The way you wrote it, via looks like a type variable
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)
and this produces a kind-mismatch error, as Additive (Square a) :: Constraint is already saturated, but you have applied it to two more arguments.
This is backwards from the shorthand form:
data T' = ...
deriving Class via T
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
add a comment |
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1 Answer
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active
oldest
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oldest
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oldest
votes
It's
deriving via (Vector2D a) instance (Additive a) => Additive (Square a)
The way you wrote it, via looks like a type variable
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)
and this produces a kind-mismatch error, as Additive (Square a) :: Constraint is already saturated, but you have applied it to two more arguments.
This is backwards from the shorthand form:
data T' = ...
deriving Class via T
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
add a comment |
It's
deriving via (Vector2D a) instance (Additive a) => Additive (Square a)
The way you wrote it, via looks like a type variable
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)
and this produces a kind-mismatch error, as Additive (Square a) :: Constraint is already saturated, but you have applied it to two more arguments.
This is backwards from the shorthand form:
data T' = ...
deriving Class via T
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
add a comment |
It's
deriving via (Vector2D a) instance (Additive a) => Additive (Square a)
The way you wrote it, via looks like a type variable
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)
and this produces a kind-mismatch error, as Additive (Square a) :: Constraint is already saturated, but you have applied it to two more arguments.
This is backwards from the shorthand form:
data T' = ...
deriving Class via T
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
It's
deriving via (Vector2D a) instance (Additive a) => Additive (Square a)
The way you wrote it, via looks like a type variable
deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)
and this produces a kind-mismatch error, as Additive (Square a) :: Constraint is already saturated, but you have applied it to two more arguments.
This is backwards from the shorthand form:
data T' = ...
deriving Class via T
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
answered Nov 21 '18 at 1:50
HTNWHTNW
10k1832
10k1832
add a comment |
add a comment |
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1
For what it is worth, I prefer your syntax with
via ...at the end to the one actually accepted by GHC, whereviacomes earlier.– chi
Nov 21 '18 at 19:50