internal-is: Generic introspector
In jeanmathieupotvin/nanocli: Create command-line interfaces easily

Check that x is of a type given by a callback function. Optionally, enforce a specific length and accept or reject NA values.

1	.is(x, isCb, targetLength = NULL, acceptNA = TRUE)

`x`	`[any]` Object to be tested.
`isCb`	`[function]` Callback function. Example: is.logical(). This is an internal parameter not visible by users.
`acceptNA`	`[logical(1)]` Can `x` contain NA values?
`targetlength`	`[NULL \| integer(1) \| double(1)]` Desired length of `x`. No length is enforced if this parameter is NULL.