We observe that in many programs, most strings are only handled as a
single unit during their lifetime. Examining real code tells us that
double quoted strings typically appear in one of the following roles:
- A DCG literal
- Although represented as a list of codes is the correct representation
for handling in DCGs, the DCG translator can recognise the literal and
convert it to the proper representation. Such code need not be modified.
- A format string
- This is a typical example of text that is conceptually not a program
identifier. Format is designed to deal with alternative representations
of the format string. Such code need not be modified.
- Getting a character code
- The construct
[X] = "a" is a commonly used template for
getting the character code of the letter’a’. ISO Prolog
defines the syntax 0'a for this purpose. Code using this
must be modified. The modified code will run on any ISO compliant Prolog
Processor.
- As argument to list predicates to operate on strings
- Here, we might see code similar to
append("name:", Rest, Codes).
Such code needs to be modified. In this particular example, the
following is a good portable alternative: phrase("name:", Codes, Rest)
- Checks for a character to be in a set
- Such tests are often performed with code such as this:
memberchk(C, "~!@#$"). This is a rather inefficient check
in a traditional Prolog system because it pushes a list of character
codes cell-by-cell onto the Prolog stack and then traverses this list
cell-by-cell to see whether one of the cells unifies with C.
If the test is successful, the string will eventually be subject to
garbage collection. The best code for this is to write a predicate as
below, which pushes nothing on the stack and performs an indexed lookup
to see whether the character code is in‘my_class’.
my_class(0'~).
my_class(0'!).
...
An alternative to reach the same effect is to use term expansion to
create the clauses:
term_expansion(my_class(_), Clauses) :-
findall(my_class(C),
string_code(_, "~!@#$", C),
Clauses).
my_class(_).
Finally, the predicate string_code/3
can be exploited directly as a replacement for the memberchk/2
on a list of codes. Although the string is still pushed onto the stack,
it is more compact and only a single entity.
