Question

Exception IO has structure:

Exception IO of {
name: string
....
...}

some other arguments that I do not understand.

Do I have to assign all these. I mean what do I do after this?

exception IO of {inputfile}

I usually define exception and then raise. but I do not even define an exception this way.

All I want to do is raise an exception if input file is not existant. What do I do here?

Thank You

Answer 1

Yes, you have to supply all three fields when creating an exception of type Io. The meanings of the fields are explained in the documentation:

This is the principal exception raised when an error occurs in the I/O subsystem. The components of Io are:

• name: The name component of the reader or writer.
• function: The name of the function raising the exception.
• cause: The underlying exception raised by the reader or writer, or detected at the stream I/O level.

Some of the standard causes are:

• OS.SysErr if an actual system call was done and failed.
  *Subscript if ill-formed arguments are given.
• BlockingNotSupported
• NonblockingNotSupported
• ClosedStream

The cause field of Io is not limited to these particular exceptions. Users who create their own readers or writers may raise any exception they like, which will be reported as the cause field of the resulting Io exception.

Note that openIn already raises an Io exception if the file does not exist (with "openIn" as the function, the filename as the name and a SysErr as the cause), so there’s no need for you to raise your own.