Non-parametric tests don’t make as many assumptions about the data, and are useful when one or more of the common statistical assumptions are violated. However, the inferences they make aren’t as strong as with parametric tests.

Commonly the above definition is called the identity function. But in fact we should think of it as a whole family of functions. We should really say that id is an identity function for all types a. In other words, for every type T you might come up with, there is an identity function called id, which is of type T -> T. This is the type-checker’s view anyway, and by turning on the RankNTypes extension we can be explicit about that in our code: {-# LANGUAGE RankNTypes #-} id :: forall a. a -> a id x = x

In other words you cannot choose the definition of a value based on its type (for now).