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DOC: Better explanation of strings and partial strings.

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Markus Triska
2022-03-06 11:05:15 +01:00
parent 0c19c56909
commit 8ad4f188f2
1 changed files with 30 additions and 17 deletions
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README.md
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@@ -280,9 +280,30 @@ in any clause of a predicate's definition.
### Strings and partial strings ### Strings and partial strings
A very compact internal representation of *strings* is one of the key
innovations of Scryer Prolog. This means that terms which appear as
lists of characters to Prolog programs are stored in packed
UTF-8 encoding by the engine.
Without this innovation, storing a list of characters in memory
would use one memory cell per character, one memory cell per
list constructor, and one memory cell for each tail that occurs
in the list. Since one memory cell takes 8 bytes on 64-bit
machines, the packed representation used by Scryer Prolog yields
an up to **24-fold reduction** of memory usage, and
corresponding reduction of memory accesses when creating and
processing strings.
Scryer Prolog's compact internal string representation makes it
ideally suited for the use case Prolog was originally developed for:
efficient and convenient text processing, especially with definite
clause grammars (DCGs) as provided by
[`library(dcgs)`](src/lib/dcgs.pl) and
[`library(pio)`](src/lib/pio.pl) to transparently apply DCGs to files.
In Scryer Prolog, the default value of the Prolog flag `double_quotes` In Scryer Prolog, the default value of the Prolog flag `double_quotes`
is `chars`, which is also the recommended setting. This means that is `chars`, which is also the recommended setting. This means that
double-quoted strings are interpreted as lists of *characters*, in the lists of characters can be written as double-quoted strings, in the
tradition of Marseille Prolog. tradition of Marseille Prolog.
For example, the following query succeeds: For example, the following query succeeds:
@@ -292,15 +313,9 @@ For example, the following query succeeds:
true. true.
``` ```
Internally, strings are represented very compactly in packed This shows that the string `"abc"`, which is represented as a sequence
UTF-8 encoding. A naive representation of strings as lists of of 3 bytes internally, appears to Prolog programs as a list of
characters would use one memory cell per character, one characters.
memory cell per list constructor, and one memory cell for
each tail that occurs in the list. Since one memory cell takes
8 bytes on 64-bit machines, the packed representation used by
Scryer Prolog yields an up to **24-fold reduction** of
memory usage, and corresponding reduction of memory accesses when
creating and processing strings.
Scryer Prolog uses the same efficient encoding for *partial* strings, Scryer Prolog uses the same efficient encoding for *partial* strings,
which appear to Prolog code as partial lists of characters. The which appear to Prolog code as partial lists of characters. The
@@ -323,13 +338,11 @@ the above example, posting <tt>Ls0&nbsp;=&nbsp;[a,b,c|Ls]</tt> yields
the exact same internal representation, and has the advantage that the exact same internal representation, and has the advantage that
only the standard predicate&nbsp;`(=)/2` is used. only the standard predicate&nbsp;`(=)/2` is used.
Definite clause grammars as provided by The efficient internal representation of strings and partial strings
[`library(dcgs)`](src/lib/dcgs.pl), and the predicates from was first proposed and explained by Ulrich Neumerkel in
[`library(lists)`](src/lib/lists.pl), are ideally suited for reasoning issues&nbsp;[#24](https://github.com/mthom/scryer-prolog/issues/24)
about strings. and&nbsp;[#95](https://github.com/mthom/scryer-prolog/issues/95), and
Scryer&nbsp;Prolog is the first Prolog&nbsp;system that implements it.
Partial strings were first proposed by Ulrich Neumerkel in issue
[#95](https://github.com/mthom/scryer-prolog/issues/95).
### Occurs check and cyclic terms ### Occurs check and cyclic terms