Motivation for default streams
Factor handbook > Input and output > Streams > Default input and output streams

Next:input-stream


Most I/O code only operates on one stream at a time. The input-stream and output-stream variables are implicit parameters used by many I/O words. Using this idiom improves code in three ways:
Code becomes simpler because there is no need to keep a stream around on the stack.
Code becomes more robust because with-input-stream and with-output-stream automatically close the streams if there is an error.
Code becomes more reusable because it can be written to not worry about which stream is being used, and instead the caller can use with-input-stream or with-output-stream to specify the source or destination for I/O operations.

For example, here is a program which reads the first line of a file, converts it to an integer, then reads that many characters, and splits them into groups of 16:
USING: continuations kernel io io.files math.parser splitting ; "data.txt" utf8 <file-reader> dup stream-readln string>number over stream-read 16 group swap dispose

This code has two problems: it has some unnecessary stack shuffling, and if either stream-readln or stream-read throws an I/O error, the stream is not closed because dispose is never reached. So we can add a call to with-disposal to ensure the stream is always closed:
USING: continuations kernel io io.files math.parser splitting ; "data.txt" utf8 <file-reader> [ dup stream-readln string>number over stream-read 16 group ] with-disposal

This code is robust, however it is more complex than it needs to be. This is where the default stream words come in; using them, the above can be rewritten as follows:
USING: continuations kernel io io.files math.parser splitting ; "data.txt" utf8 <file-reader> [ readln string>number read 16 group ] with-input-stream

An even better implementation that takes advantage of a utility word:
USING: continuations kernel io io.files math.parser splitting ; "data.txt" utf8 [ readln string>number read 16 group ] with-file-reader