Missing method in type class instance [GHC06201]
Flag: Wmissingmethods
Enabled by default
This warning means that a type class instance is missing some required method implementations.
Examples
No explicit implementation for method instance
Error Message
A.hs:7:10: warning: [GHC06201] [Wmissingmethods]
• No explicit implementation for
‘multiply’
• In the instance declaration for ‘Calculator Int’

7  instance Calculator Int where
Explanation
The type class Calculator
requires you to implement two methods: add
and multiply
.
However, the example instance instance Calculator Int
only implements the method add
and not multiply
To fix this, implement the method multiply
!
A.hs
Before
module A where
class Calculator a where
add :: a > a > a
multiply :: a > a > a
instance Calculator Int where
add a b = a + b
After
module A where
class Calculator a where
add :: a > a > a
multiply :: a > a > a
instance Calculator Int where
add a b = a + b
multiply a b = a * b
No explicit implementation for method instance 2
Error Message
A.hs:8:10: warning: [GHC06201] [Wmissingmethods]
• No explicit implementation for
either ‘<*>’ or ‘liftA2’
• In the instance declaration for ‘Applicative Box’

8  instance Applicative Box where
 ^^^^^^^^^^^^^^^
Explanation
The type class Applicative
requires you to implement at least two methods:
pure
, andliftA2
ormultiply
(can also be both).
Such or
constraints can be expressed via the {# MINIMAL #}
pragma.
To fix this, either implement the method liftA2
or <*>
. For this example, we implemented <*>
, but either or both is fine, too.
A.hs
Before
module A where
data Box t = SomeBox t
instance Functor Box where
fmap f (SomeBox a) = SomeBox (f a)
instance Applicative Box where
pure a = SomeBox a
After
module A where
data Box t = SomeBox t
instance Functor Box where
fmap f (SomeBox a) = SomeBox (f a)
instance Applicative Box where
pure a = SomeBox a
SomeBox f <*> SomeBox a = SomeBox (f a)