T7869.hs:3:12: error:
• Couldn't match type ‘b’ with ‘b1’
because type variable ‘b1’ would escape its scope
This (rigid, skolem) type variable is bound by
an expression type signature:
[a1] -> b1
at T7869.hs:3:20-27
Expected type: [a1] -> b1
Actual type: [a] -> b
• In the expression: f x
In the expression: (\ x -> f x) :: [a] -> b
In an equation for ‘f’: f = (\ x -> f x) :: [a] -> b
• Relevant bindings include f :: [a] -> b (bound at T7869.hs:3:1)