Interface Wr describes the principle type Wr.T (or writer) is an output stream. The basic operation on a writer is "PutChar", which extends a writer's character sequence by one character. Some writers (called seekable writers) also allow overwriting in the middle of the sequence. For example, writers to random access files are seekable, but writers to terminals and sequential files are not. There are a variety of writers available in Modula-3 libraries, for creating writers to text, or over the network.
See the description of Wr interface for more information.