`Could not deduce: Show <+> '[]`

[yaitskov]

Hi,

<+> type operator need a fix. In GHC 8.10 and GHC 9.2.4:

Definition is compiled, but function invocation gives a type error:

sp :: Show <+> [a, b] => a -> b -> String ; sp a b = show a <> show b

> sp () ()

<interactive>:35:1: error:
    • Could not deduce: Show <+> '[] arising from a use of ‘sp’
    • In the expression: sp () ()
      In an equation for ‘it’: it = sp () ()

Extending type family with follow member solve the problem:

type instance (<+>) _ '[] = (() :: Constraint)