No instance arising [GHC39999]
This error happens when an expression in the code has a type constraint (e.g. Eq a => a > a
) requiring an instance of a type class but GHC can‘t find it.
This most commonly happens for two reasons: Either the type is too polymorphic (i.e. general) e.g. forall a. a
and there is no type signature declaring that the type should have the instance. Or the type is specific e.g. Bool
, but there is no instance for the desired type class defined for this type.
Examples
A usage of (==) on a data type which doesn‘t have an instance for Eq.
Error message
Main.hs:4:15: error: [GHC39999]
• No instance for (Eq Foo) arising from a use of ‘==’
• In the expression: foo == A
In an equation for ‘isA’: isA foo = foo == A

4  isA foo = foo == A
 ^^
Explanation
The operator ==
is only defined on types which have an instance for the Eq
type class. The data type Foo does not have an instance. So GHC doesn‘t know how to compare values of Foo
.
To make this example compile we can create an Eq
instance by using a derive
statement.
Main.hs
data Foo = A  B
isA :: Foo > Bool
isA foo = foo == A
data Foo = A  B deriving Eq
isA :: Foo > Bool
isA foo = foo == A
A usage of `+` on a too polymorphic variable.
Error message
Main.hs:2:11: error: [GHC39999]
• No instance for (Num a) arising from a use of ‘+’
Possible fix:
add (Num a) to the context of
the type signature for:
add :: forall a. a > a > a
• In the expression: x + y
In an equation for ‘add’: add x y = x + y

2  add x y = x + y
 ^
Explanation
The operator +
is only defined on types which have an instance of the Num
type class. If the function has the signature a > a > a
someone could e.g. pass in a String
for a
and GHC wouldn‘t know how to add two Strings. Too fix it, like the compiler suggests, we add the Num a =>
constraint to the type signature to make sure that all users of add
provide a suitable Num
instance.
Main.hs
add :: a > a > a
add x y = x + y
add :: Num a => a > a > a
add x y = x + y