operator
source expression || source expression
The infix operator || concatenates its two arguments. It can be applied to string, string source, or markup source arguments. The result is a string source if both arguments
are of the string or string source type. For example, the expression "Input file: " ||
file "input.txt" has the type string source. If either argument is a markup source, as
in the expression #content || "%n", the result type is markup source.
The || operator consumes its arguments lazily: it starts consuming its second argument only when
the first argument is exhausted. This can be important in examples like the following, when the second argument
is a string source function:
define string source function generate-instance as output "<" || #doctype || ">" || #main-input || "</" || #doctype || ">" process do sgml-parse document scan file "input.dtd" || generate-instance output "%c" done
If the function generate-instance was forced to run before the DTD was parsed, it could not obtain the document type it needs to generate the top-level element.
Another thing to note about this example is that the body of generate-instance consists of a single
output action with six || operators. Because the concatenation operator is lazy, this definition
behaves the same as the following one with multiple output actions:
define string source function generate-instance as output "<" || #doctype || ">" output #main-input output "</" || #doctype || ">"