(with-output-to-file str thunk)
String should be a string naming a file, and proc should be a procedure of no arguments. For with-input-from-file, the file should already exist; for with-output-to-file, the effect is unspecified if the file already exists. The file is opened for input or output, an input or output port connected to it is made the default value returned by current-input-port or current-output-port (and is used by (read), (write obj), and so forth), and the thunk is called with no arguments. When the thunk returns, the port is closed and the previous default is restored. With-input-from-file and with-output-to-file return(s) the value(s) yielded by thunk. If an escape procedure is used to escape from the continuation of these procedures, their behavior is implementation dependent.
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