Quantified constraints

This wiki page summarises the state of play on the idea of allowing quantification in class constraints. For example

data Rose f a = Branch a (f (Rose f a))

instance (Eq a, forall b. (Eq b) => Eq (f b))
       => Eq (Rose f a)
  where ...

The new bit is the forall in the context of the instance declaration. This is allowed in GHC 8.6 and later using the QuantifiedConstraints extension.

Here are some resources

• The GHC Proposal discussing quantified constraints

• Derivable type classes, Section 7, where the idea was first proposed (I think).

• #2893 (closed), a ticket about the idea

• Quantified class constraints, a Haskell 2017 paper that works out the idea in some detail, and a Reddit thread about it.

• An old haskell.org Wiki page about it

• A Libraries thread (Dec 18).

Status

See the QuantifiedConstraints label for tickets.