[source]

compiler/iface/BuildTyCl.hs

Note [Class newtypes and equality predicates]

[note link]

Consider

class (a ~ F b) => C a b where

op :: a -> b

We cannot represent this by a newtype, even though it’s not existential, because there are two value fields (the equality predicate and op. See #2238

Moreover,

class (a ~ F b) => C a b where {}

Here we can’t use a newtype either, even though there is only one field, because equality predicates are unboxed, and classes are boxed.