Missing FunctionalDependencies [GHC-15708]
The Functional Dependencies extension enables dependencies in multiparameter class declarations.
When the extension is missing, the syntax for specifying dependencies is not recognized.
Examples
Missing FunctionalDependencies language extension
Cannot specify functional dependencies for a class - FunctionalDependencies language extension is missing.
Error Message
FunDepsDisabled.hs:1:1: error: [GHC-15708]
• Fundeps in class ‘Convert’
• In the class declaration for ‘Convert’
Suggested fix: Perhaps you intended to use FunctionalDependencies
|
1 | class Convert a b | a -> b where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...FunDepsDisabled.hs
Before
module FunDepsDisabled where
class Convert a b | a -> b where
convert :: a -> b
After
{-# LANGUAGE FunctionalDependencies #-}
module FunDepsDisabled where
class Convert a b | a -> b where
convert :: a -> b