However, this approach suffers from two problems. One is simply that it’s error prone—if you forget the **CLOSE**, the code will leak a file handle every time it runs. The other—and more significant—problem is that there’s no guarantee you’ll get to the **CLOSE**. For instance, if the code prior to the **CLOSE** contains a **RETURN** or , you could leave the **LET** without closing the stream. Or, as you’ll see in Chapter 19, if any of the code before the **CLOSE** signals an error, control may jump out of the **LET** to an error handler and never come back to close the stream.

The forms in body-forms are evaluated with stream-var bound to a file stream opened by a call to **OPEN** with open-arguments as its arguments. **WITH-OPEN-FILE** then ensures the stream in stream-var is closed before the **WITH-OPEN-FILE** form returns. Thus, you can write this to read a line from a file:

You’ll probably use **WITH-OPEN-FILE** for 90-99 percent of the file I/O you do—the only time you need to use raw **OPEN** and calls is if you need to open a file in a function and keep the stream around after the function returns. In that case, you must take care to eventually close the stream yourself, or you’ll leak file descriptors and may eventually end up unable to open any more files.