Data frames carry application-layer and/or extension-layer data. The
opcode determines the interpretation of the data:
Text
The "Payload data" is text data encoded as UTF-8. Note that a
particular text frame might include a partial UTF-8 sequence;
however, the whole message MUST contain valid UTF-8. Invalid
UTF-8 in reassembled messages is handled as described in
Section 8.1.
Binary
The "Payload data" is arbitrary binary data whose interpretation
is solely up to the application layer.
T это generic параметр. В данном контексте это тип фильтруемого значения.(source: T): R которая еще и унарная функция итд. Это всё иерархия типов для того что бы можно было композировать друг с другом функции по их "смыслу" а не сигнатуре.
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