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Here we talk about Guile modules that are distributed with combine.
At the moment, those are limited to date processing.
In addition, the file ‘util.scm’ in the distribution contains a few functions I have found handy. They are not documented here, and the file doesn’t get installed automatically.
| 4.2.1 Calendar Functions | ||
| 4.2.2 Calendar Reference | ||
| 4.2.3 Calendar Parsing |
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Included in the combine package are two Guile modules to work with dates
from a number of calendars, both obscure and common. The basis for them is
the set of calendar functions that are shipped with Emacs.
The reason that these functions deserve special notice here is that date comparisons are a common type of comparison that often cannot be made directly on a character string. For example I might have trouble knowing if "20030922" is the same date as "22 September 2003" if I compared strings; however, comparing them as dates allows me to find a match. We can even compare between calendars, ensuring that "1 Tishri 5764" is recognized as the same date as "20030927".
The calendar module can be invoked as (use-modules (combine_scm calendar)).
It provides functions for converting from a variety of calendars to and from
and absolute date count, whose 0-day is the imaginary date 31 December 1 B.C.
In the functions, the absolute date is treated as a single number, and the
dates are lists of numbers in (month day year) format unless otherwise
specified.
The calendar functions are as follow:
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Here are some variables that can be used as references to get names associated with the numbers that the date conversion functions produce for months.
An associative list giving the weekdays in the Gregorian calendar in a variety of languages. Each element of this list is a list composed of a 2-letter language code (lowercase) and a list of 7 day names.
An associative list giving the months in the Gregorian calendar in a variety of languages. Each element of this list is a list composed of a 2-letter language code (lowercase) and a list of 12 month names.
A list of the months in the Islamic calendar.
A list of the months in the standard Hebrew calendar.
A list of the months in the leap year Hebrew calendar.
A list of the months in the French Revolutionary calendar.
A list of the months in the French Revolutionary calendar, using multibyte codes to represent the accented characters.
A list of the days in the French Revolutionary calendar.
A list of the special days (non weekdays) in the French Revolutionary calendar, using multibyte codes to represent the accented characters.
A list of the special days (non weekdays) in the French Revolutionary calendar.
A list of the months in the Coptic calendar.
A list of the months in the Ethiopic calendar.
A list of the months in the Persian calendar.
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The calendar parsing module can be invoked as (use-modules (combine_scm parse)).
The most useful function in the module is parse-date. It takes as arguments
a date string and an output format. The date string is parsed as well as possible
in descending order of preference for format in case of ambiguity. The function
returns the date triplet (or other such representation) suggested by the format
string.
The supported format strings are the words in the function names of the form
calendar-xxxx-from-absolute that would take the place of the xxxx.
See section Calendar Functions, for more information.
The parsing of the date string depends on the setting of a couple of variables. Look inside the file ‘parse.scm’ for details. The list parse-date-expected-order lists the order in which the parser should look for the year, month, and day in case of ambiguity. The list parse-date-method-preference give more general format preferences, such as 8-digit, delimited, or a word for the month and the expected incoming calendar.
Here are a few examples of passing a date and putting it out in some formats:
guile> (use-modules (combine_scm parse)) guile> (parse-date "27 September 2003" "gregorian") (9 27 2003) guile> (parse-date "27 September 2003" "julian") (9 14 2003) |
The 13 day difference in the calendars is the reason that the Orthodox Christmas is 2 weeks after the Roman Catholic Christmas.
guile> (parse-date "27 September 2003" "hebrew") (7 1 5764) |
Note that the Hebrew date is Rosh HaShannah, the first day of the year 5764. The reason that the month is listed as 7 rather than 1 is inherited from the Emacs calendar implementation. Using the month list in calendar-hebrew-month-name-array-common-year or calendar-hebrew-month-name-array-leap-year correctly gives "Tishri", but since the extra month (in years that have it) comes mid-year, the programming choice that I carried forward was to cycle the months around so that the extra month would come at the end of the list.
guile> (parse-date "27 September 2003" "islamic") (7 30 1424) guile> (parse-date "27 September 2003" "iso") (39 6 2003) |
This is the 6th day (Saturday) of week 39 of the year.
guile> (parse-date "27 September 2003" "mayan-long-count") (12 19 10 11 7) |
I won’t get into the detail, but the five numbers reflect the date in the Mayan calendar as currently understood.
Generally, I’d recommend using the more specific functions if you are sure of the date format you expect. For comparing dates, I would further recommend comparing the absolute day count rather than any more formatted format.
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