Sha256: 9667be1ee3aebdb523da926736b7644463d601ba29c44e00f2eeb3080b41af81
Contents?: true
Size: 1.8 KB
Versions: 7
Compression:
Stored size: 1.8 KB
Contents
module SPARQL; module Algebra
class Operator
##
# The SPARQL `sample` set function.
#
# [127] Aggregate::= ... | 'SAMPLE' '(' 'DISTINCT'? Expression ')'
#
# @example SPARQL Grammar
# PREFIX : <http://example/>
#
# SELECT ?w (SAMPLE(?v) AS ?S)
# {
# ?s :p ?v .
# OPTIONAL { ?s :q ?w }
# }
# GROUP BY ?w
#
# @example SSE
# (prefix ((: <http://example/>))
# (project (?w ?S)
# (extend ((?S ??.0))
# (group (?w) ((??.0 (sample ?v)))
# (leftjoin
# (bgp (triple ?s :p ?v))
# (bgp (triple ?s :q ?w))))) ))
#
# @see https://www.w3.org/TR/sparql11-query/#defn_aggSample
class Sample < Operator
include Aggregate
NAME = :sample
def initialize(*operands, **options)
raise ArgumentError,
"sample operator accepts at most one argument with an optional :distinct" if
(operands - %i{distinct}).length != 1
super
end
##
# Sample is a set function which returns an arbitrary value from the multiset passed to it.
#
# @param [Enumerable<Array<RDF::Term>>] enum
# enum of evaluated operand
# @return [RDF::Term] An arbitrary term
# @raise [TypeError] If enum is empty
def apply(enum, **options)
enum.detect(lambda {raise TypeError, "Sampling an empty multiset"}) {|e| e.first}.first
end
##
#
# Returns a partial SPARQL grammar for this operator.
#
# @return [String]
def to_sparql(**options)
distinct = operands.first == :distinct
args = distinct ? operands[1..-1] : operands
"SAMPLE(#{'DISTINCT ' if distinct}#{args.to_sparql(**options)})"
end
end # Sample
end # Operator
end; end # SPARQL::Algebra
Version data entries
7 entries across 7 versions & 1 rubygems