Several implementions of a single URL abstraction

[aadcg]

As to ensure that convert strings to quri:uri objects, we have defined:

• url methods for strings and quri:uri
• ensure-url
• string->url
• strings->urls

Also we have defined the type url-designator, as to accommodate that a URL may be of type quri:uri or string, but that seems a bit premature.

[aadcg]

Partially addressed in 94802b9 and bdcf23e.

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A deeper refactor is needed but I'll postpone it since it is a long refactor that may break some functionality (special care is needed).

The idea is simple. Define the routine below and delete the url methods that specialize over string and quri:uri.

(defun coerce-to-url (url-designator)
  "Ensure URL-DESIGNATOR is of type `quri:uri'."
  (typecase url-designator
    (string (quri:uri url-designator))
    (quri:uri url-designator)))

Redefine the url-designator type as below.

(deftype url-designator ()
  "A URL designator type is either a string or a `quri:uri'."
  `(or string quri:uri))

These 2 ideas will enable refactoring the following odd idiom,

(defun url-util (url) (let ((url (url url))) url))

into:

(defun url-util (url-designator) (let ((url (coerce-to-url url-designator))) url))