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(elisp)Stream Type

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Stream Type
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   A "stream" is an object that can be used as a source or sink for
characters--either to supply characters for input or to accept them as
output.  Many different types can be used this way: markers, buffers,
strings, and functions.  Most often, input streams (character sources)
obtain characters from the keyboard, a buffer, or a file, and output
streams (character sinks) send characters to a buffer, such as a
`*Help*' buffer, or to the echo area.

   The object `nil', in addition to its other meanings, may be used as
a stream.  It stands for the value of the variable `standard-input' or
`standard-output'.  Also, the object `t' as a stream specifies input
using the minibuffer (Note: Minibuffers) or output in the echo area
(Note: The Echo Area).

   Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.

   Note: Read and Print, for a description of functions related to
streams, including parsing and printing functions.