open

(open filename &optional direction element-type)

Opens a file and returns a stream. The optional direction must be the literal keyword :input (the default -- open for reading) or :output (create or truncate, open for writing); it must be a literal so the compilers can pick the file mode statically. The optional element type -- also a literal -- selects the stream kind: 'character (the default) opens a text stream for read/read-line/write-line, and '(unsigned-byte 8) opens a binary stream for read-byte/write-byte/read-sequence/write-sequence. The returned stream is an opaque handle (an index into a stream table on the interpreter/JVM, the WASI file descriptor on WASM) valid only within the producing run, and should be passed to the matching read/write functions and then close. On WASM the path is resolved against the first preopened directory, so run with --dir. Prefer with-open-file, which closes the stream automatically.

(let ((s (open "data.txt")))
  (print (read-line s))
  (close s))

This opens data.txt for input, reads its first line, and closes the stream. Passing :output instead would create or truncate the file for writing; (open "data.bin" :input '(unsigned-byte 8)) opens the same kind of handle in binary mode.