/usr/local/lib/swipl/library/ext/semweb/semweb/rdf11.pl
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 rdf_canonical_literal(++In, -Literal) is det
Transform a relaxed literal specification as allowed for rdf_assert/3 into its canonical form. The following Prolog terms are translated:
Prolog TermDatatype IRI
floatxsd:double
integerxsd:integer
stringxsd:string
true or falsexsd:boolean
date(Y,M,D)xsd:date
date_time(Y,M,D,HH,MM,SS)xsd:dateTime
date_time(Y,M,D,HH,MM,SS,TZ)xsd:dateTime
month_day(M,D)xsd:gMonthDay
year_month(Y,M)xsd:gYearMonth
time(HH,MM,SS)xsd:time

For example:

?- rdf_canonical_literal(42, X).
X = 42^^'http://www.w3.org/2001/XMLSchema#integer'.