{-
I am trying to understand and provide a *simplified* explanation of instance contexts and their relationship with class hierarchies.
I use the example from [1]. Are the following two sentences and annotated code a reasonable explanation?
" When instantiating an instance I of C, its context must be at the same level or lower than the context of any instance of any super-class of C.
The purpose of this rule is to guarantee that the required super-class methods exist."
[1] 4.3.2 Instance Declarations
http://www.haskell.org/onlinereport/haskell2010/haskellch4.html
Class hierarchy
Eq1 Show1 Foo
\ / |
\ / |
Num1 Bar
|
|
Num2
-}
class Foo a where
class Show1 a where
class Foo a => Bar a where
class Eq1 a where
-- Eq1 and Show1 are super-classes of Num1 and Num2.
class (Eq1 a, Show1 a) => Num1 a
class Num1 a => Num2 a
-- We must make an instance of Foo [a], before we can have instance Bar [a]
instance (Num1 a) => Foo [a] where
-- But that instance of Foo [a] depends on a being a member of Num1
-- Hence Bar[a] can only be defined if their exists Num1 a
-- But Eq1 & Show1 are super classes of Num1
-- The following context causes an *error*, because the context is weaker than required
-- instance (Eq1 a, Show1 a) => Bar [a] where
-- But this is OK
instance (Num1 a) => Bar [a] where
-- Also, this would be OK
-- instance (Num2 a) => Bar [a] where
I am trying to understand and provide a *simplified* explanation of instance contexts and their relationship with class hierarchies.
I use the example from [1]. Are the following two sentences and annotated code a reasonable explanation?
" When instantiating an instance I of C, its context must be at the same level or lower than the context of any instance of any super-class of C.
The purpose of this rule is to guarantee that the required super-class methods exist."
[1] 4.3.2 Instance Declarations
http://www.haskell.org/onlinereport/haskell2010/haskellch4.html
Class hierarchy
Eq1 Show1 Foo
\ / |
\ / |
Num1 Bar
|
|
Num2
-}
class Foo a where
class Show1 a where
class Foo a => Bar a where
class Eq1 a where
-- Eq1 and Show1 are super-classes of Num1 and Num2.
class (Eq1 a, Show1 a) => Num1 a
class Num1 a => Num2 a
-- We must make an instance of Foo [a], before we can have instance Bar [a]
instance (Num1 a) => Foo [a] where
-- But that instance of Foo [a] depends on a being a member of Num1
-- Hence Bar[a] can only be defined if their exists Num1 a
-- But Eq1 & Show1 are super classes of Num1
-- The following context causes an *error*, because the context is weaker than required
-- instance (Eq1 a, Show1 a) => Bar [a] where
-- But this is OK
instance (Num1 a) => Bar [a] where
-- Also, this would be OK
-- instance (Num2 a) => Bar [a] where
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