uninformative diagnostic for an unknown variable
in ghci 8.6.5 this one-line input
take 10 $ x^2evokes this complicated (and misdirecting) diagnostic
• Non type-variable argument in the constraint: Num [a]
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. Num [a] => [a]when the root cause is
variable not in scope: x