open

(open filename &optional direction element-type)

Opens a file and returns a stream. The optional direction is :input (the default -- open for reading) or :output (create or truncate, open for writing). The keyword-argument shape (open filename :direction :output :if-exists :append) opens for writing WITHOUT truncating, so every write lands at the end of an existing file; the other :if-exists/:if-does-not-exist/:external-format values are accepted only where they name the native behavior already (:supersede, :create/:error, :utf-8/:default). The optional element type selects the stream kind: 'character (the default) opens a text stream for read/read-line/write-line, and '(unsigned-byte 8) -- or the unsized 'unsigned-byte -- opens a binary stream for read-byte/write-byte/read-sequence/write-sequence. In the keyword shape an option value may be COMPUTED ((open path :direction dir :element-type type)): it is read when the call runs and dispatched onto the matching literal shape, which is what lets a portable wrapper take its options as arguments. A literal value is resolved at compile time exactly as before, and a value outside the supported set signals an error when the call runs. 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 preopened directories -- a relative path against the first one, an absolute path against the preopened directory whose name is its longest prefix -- 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.