Package "http_date"

Title:	RFC 9110 HTTP date parsing and formatting
Rating:	Not rated. Create the first rating!
Latest version:	0.1.0
SHA1 sum:	1b574a7e6e8db4ccc6ad7c57c041d1baba4ecbdd
Author:	Bikal Gurung <gbikal@gmail.com>
Home page:	https://github.com/bikallem/http_date
Download URL:	https://github.com/bikallem/http_date/archive/v0.1.0.zip

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Details by download location

Version	SHA1	#Downloads	URL
0.1.0	1b574a7e6e8db4ccc6ad7c57c041d1baba4ecbdd	1	https://github.com/bikallem/http_date.git

http_date

Parse and format HTTP date values in SWI-Prolog, as defined in RFC 9110 section 5.6.7.

An HTTP date appears in one of three formats.

Format	Example
IMF-fixdate	`Sun, 06 Nov 1994 08:49:37 GMT`
RFC 850	`Sunday, 06-Nov-94 08:49:37 GMT`
asctime	`Sun Nov 6 08:49:37 1994`

One grammar, http_date//2, defines all three. decode/2 and encode/2 run that same grammar in opposite directions. The two directions cannot disagree.

Install

?- pack_install('.').

Use

?- use_module(library(http_date)).

?- decode("Sun, 06 Nov 1994 08:49:37 GMT", D).
D = http_date(imf_fixdate, datetime(sun, date(1994, 11, 6), time(8, 49, 37))).

?- encode(http_date(asctime, datetime(sun, date(1994,11,6), time(8,49,37))), S).
S = "Sun Nov  6 08:49:37 1994".

A decoded value is the term http_date(Format, datetime(Day, Date, Time)). Format is one of imf_fixdate, rfc850 or asctime. Unify against it to ask which format you have, or call is_imf_fixdate/1, is_rfc850/1 or is_asctime/1.

The grammar is exported. You can call it from a larger grammar and avoid an intermediate string.

last_modified(Format, DateTime) -->
    `Last-Modified: `, http_date(Format, DateTime).

Shape and meaning are separate checks

decode/2 checks shape only. It accepts the three layouts, rejects trailing data, and rejects an unknown day name or month. It does not make sure that the numbers name a real instant. For example, `Sun, 99 Nov 1994 08:49:37 GMT` decodes to date(1994, 11, 99).

valid_http_date/1 checks meaning. It rejects a day the month does not have. It rejects 29 February in a year that is not a leap year. It also rejects an hour above 23, or a minute or second above 59.

?- decode(Text, D), valid_http_date(D).

strict_http_date/1 adds the day of the week. It calls valid_http_date/1, then checks the day name against the calendar with consistent_dayname/1. For the RFC 850 format it skips that check, because a two digit year does not say which century it means.

The split is deliberate. RFC 9110 lets a recipient accept a date whose day name is wrong. The parser stays permissive. The caller decides how much to demand.

Timestamps

http_date_stamp/2 converts a value to a Unix timestamp, and stamp_http_date/3 converts a timestamp back. Both treat the value as GMT.

?- decode("Sun, 06 Nov 1994 08:49:37 GMT", D), http_date_stamp(D, Stamp).
Stamp = 784111777.0.

?- get_time(Now), stamp_http_date(Now, imf_fixdate, D), encode(D, S).

Call valid_http_date/1 before you convert. date_time_stamp/2 normalizes an impossible date instead of refusing it. 31 February becomes 3 March.

An RFC 850 value holds a two digit year. http_date_stamp/2 reads that year literally. Sunday, 06-Nov-94 gives a timestamp in the year 94. If you need a century, map the year yourself.

Tests

swipl -q -g run_tests -t halt prolog/http_date.pl

The suite runs 127 cases. The round trip test reads the same tables the grammar reads. It generates one case for every day and month pair.

Development

The repository carries a Nix flake. `nix develop` gives you a shell with SWI-Prolog on the path. If you use direnv, run `direnv allow` once. The shell then loads on entry.

Contents of pack "http_date"

Pack contains 6 files holding a total of 12.6K bytes.