Illegal typeclass instance [GHC93557]
Using the language version Haskell 2010, only certain types are allowed as instances of a type class. Every type class instance in Haskell 2010 has the following form:
instance C t where
Here, C
is the name of a type class, such as Eq
, Show
, Read
or Functor
, and t
is a type.
But only a subset of types t
is allowed to appear in typeclass instances.
These types must all have the form T
or T a ... b
, where T
has to be the name of a type introduced by a data or newtype declaration, and the arguments a
to b
all have to be type variables.
For example, Maybe a
is allowed as a type in an instance, because the data type Maybe
instantiates T
and a
is a type variable, but
Maybe Intis not allowed, since
Int` is not a type variable.
Examples
Cannot implement a typeclass for type synonyms by default
The programmer defined a data type RGB
, and a type synonym T
, and then tried to implement an instance of the type class Eq
for T
.
In the language version Haskell 2010 this is not allowed, since all type class instances have to start with a type constructor.
This problem can be fixed by implementing the typeclass instance directly for the type RGB
instead of the type synonym T
.
messages/GHC93557/illegalSynonymInstance/before/IllegalSynonymInstance.hs:8:10: error: [GHC93557]
• Illegal instance declaration for ‘Eq T’:
All instance types must be of the form (T t1 ... tn)
where T is not a synonym.
• In the instance declaration for ‘Eq T’
Suggested fix: Perhaps you intended to use TypeSynonymInstances

8  instance Eq T where
 ^^^^
IllegalSynonymInstance.hs
{# LANGUAGE Haskell2010 #}
module IllegalSynonymInstance where
data RGB = R  G  B
type T = RGB
instance Eq T where
R == R = True
G == G = True
B == B = True
_ == _ = False
{# LANGUAGE Haskell2010 #}
module IllegalSynonymInstance where
data RGB = R  G  B
instance Eq RGB where
R == R = True
G == G = True
B == B = True
_ == _ = False