6.10.1. Loosening restrictions on class contexts

FlexibleContexts

Since:

6.8.1

Status:

Included in GHC2021

Remove the type-variable restriction on class contexts.

The FlexibleContexts extension lifts the Haskell 98 restriction that the type-class constraints (anywhere they appear) must have the form (class type-variable) or (class (type-variable type1 type2 … typen)). With FlexibleContexts these type signatures are perfectly okay:

g :: Eq [a] => ...
g :: Ord (T a ()) => ...

This extension does not affect equality constraints in an instance context; they are permitted by TypeFamilies or GADTs.

Note that FlexibleContexts affects usages of class constraints, in type signatures and other contexts. In contrast, FlexibleInstances loosens a similar restriction in place when declaring a new instance.