tcfail099.hs:9:20: error:
• Couldn't match expected type ‘a’ with actual type ‘t’
because type variable ‘a’ would escape its scope
This (rigid, skolem) type variable is bound by
a pattern with constructor: C :: forall a. (a -> Int) -> DS,
in an equation for ‘call’
at tcfail099.hs:9:7-9
• In the first argument of ‘f’, namely ‘arg’
In the expression: f arg
In an equation for ‘call’: call (C f) arg = f arg
• Relevant bindings include
arg :: t (bound at tcfail099.hs:9:12)
f :: a -> Int (bound at tcfail099.hs:9:9)
call :: DS -> t -> Int (bound at tcfail099.hs:9:1)