Constraints for Recursive Types

Question

I have a type data A a = B (a (A a)). How can I put a constraint on the type var a in function something :: Eq (a b) => A a -> SomeType?

Presumably you actually use Eq (a (A a)) (not Eq (a b) for some arbitrary b) in the implementation of something, no? — Daniel Wagner
– Daniel Wagner, Commented Aug 17, 2019 at 17:05

chi · Accepted Answer · 2019-08-18 07:33:41Z

6

It's not completely clear to me what you want to achieve, but this compiles:

{-# LANGUAGE QuantifiedConstraints, StandaloneDeriving, UndecidableInstances #-}

data A a = B (a (A a))

deriving instance (forall t. Eq t => Eq (a t)) => Eq (A a)

something :: (forall t. Eq t => Eq (a t)) => A a -> String
something x 
   | x==x      = "hello"
   | otherwise = "world"

The trick here is to require that Eq (a t) holds for any possible t. That requires QuantifiedConstraints.

Of course, you can also use a more modest approach and require instead

something :: Eq (a Bool) => A a -> String

but that won't allow you to use == on the argument.

Alternatively,

something :: Eq (A a) => A a -> String

should work, even if it triggers a warning.

edited Aug 18, 2019 at 7:33

answered Aug 17, 2019 at 17:09

chi

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2 Comments

William Rusnack Over a year ago

I have not seen QuantifiedConstraints before, so thanks. That works for me

bradrn Over a year ago

@WilliamRusnack It was only introduced about a year ago, in GHC 8.6.1.

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