I am reading Theorems for free! by Philip Wadler which is a paper about how to derive theorems from the type signature of a function.
Parametricity is just a reformulation of Reynolds’ abstraction theorem: terms evaluated in related environments yield related values.
What is an environment in this case ? More generally, how do I interpret the statement in bold ?
I think I understand the parametricity thing correctly but somehow I cannot map this sentence to something more intuitive.