Forall types in a record don't work the same as non-records.

[ChrisPenner]

I attempted to define:

structural type Functor f = 
  {map : forall a b. (a -> b) -> f a -> f b }

Which failed with:

  I found a value  of type:  (a1 -> b1) -> 𝕣
  where I expected to find:  (a -> b) -> f a -> f b

      2 |   {map : forall a b. (a -> b) -> f a -> f b }

    from right here:

      2 |   {map : forall a b. (a -> b) -> f a -> f b }

Reconfiguring it as not-a-record works:

structural type Functor f = 
  Functor (forall a b. (a -> b) -> f a -> f b)

[pchiusano]

This is a dupe of some issue that I can't locate. It's caused because the record accessors have inferred types, and higher-rank types are never inferred. (A record accessor can have pretty complicated type since it's allowed to change type parameters, so it's not simple to figure out what type we should ascribe to them)

I think more and more we should just do proper record types.