Add regex types to types module

Current state

In the types module the type of objects can be checked. This is not restricted to Prolog's definition of types: at least for dicts required and optional keys (with type restrictions) may be defined.

Suggestion

The regex type would extend this idea to strings: the value must not only be of type string but its content must satisfy additional requirements. Additionally, a regex type for atoms or general text is desired.

Rationale

We have decided, that absolute paths of files and directories may not end in a slash unless it is the root directory '/'. Without the regex type, this restriction must be enforced after type checking. However for this, a mechanisms very similar to type checking are required. For example in set_bg:

• if we have a single dict:
  • noting which values are concerned (probably only abs_path),
  • extracting the value
  • checking the regex
  • if the check fails: throw exception
• if we have a list of dicts:
  • iterate over all elements
  • if we then have a dict, apply the steps from above

It is easier, less error prone, and more encapsulated to include this facility into types.

Technical realisation

The actual checking of the type might be simple as there is a regex library in Prolog. Taking strings as example, the type definition could look like

• regex(string, RegEx) where RegEx must be a valid regular expression from Prolog's regex library or
• string(RegEx) where RegExis defined as above.

The second option would convey the notion that this is a refinement of the string type more strongly. This is analogous to dict vs. dict/1 and dict/2 and list vs list/1 already realized in types.