5.2.3 Why has the representation of double quoted text changed?

Prolog defines two forms of quoted text. Traditionally, single quoted text is mapped to atoms while double quoted text is mapped to a list of character codes (integers) or characters (atoms of length 1). Representing text using atoms is often considered inadequate for several reasons:

• It hides the conceptual difference between text and program symbols. Where content of text often matters because it is used in I/O, program symbols are merely identifiers that match with the same symbol elsewhere. Program symbols can often be consistently replaced, for example to obfuscate or compact a program.

• Atoms are globally unique identifiers. They are stored in a shared table. Volatile strings represented as atoms come at a significant price due to the required cooperation between threads for creating atoms. Reclaiming temporary atoms using Atom garbage collection is a costly process that requires significant synchronisation.

• Many Prolog systems (not SWI-Prolog) put severe restrictions on the length of atoms or the maximum number of atoms.

Representing text as lists, be it of character codes or characters, also comes at a price:

• It is not possible to distinguish (at runtime) a list of integers or atoms from a string. Sometimes this information can be derived from (implicit) typing. In other cases the list must be embedded in a compound term to distinguish the two types. For example, s("hello world") could be used to indicate that we are dealing with a string.

  Lacking runtime information, debuggers and the toplevel can only use heuristics to decide whether to print a list of integers as such or as a string (see portray_text/1).

  While experienced Prolog programmers have learned to cope with this, we still consider this an unfortunate situation.

• Lists are expensive structures, taking 2 cells per character (3 for SWI-Prolog in its current form). This stresses memory consumption on the stacks while pushing them on the stack and dealing with them during garbage collection is unnecessarily expensive.