System.IO.Utf8.Internal

What to do with the encoding of the handle.

We return new encoding as string to simplify testing, because, it turns out, when you make an encoding with some name, the resulting encoding can have a different name.

The second constructor also contains the old encoding as a proof that the encoding is set to a new one only if there was a previous one in the first place. You probably think I am crazy, but I am not, it is just the simplest way to make the hSetBestUtf8Enc easy to write.

Pure version of chooseBestEnc.

This function is not actually used in the library. It exists only for documentation purposes to demonstrate the logic. It is also used in tests to verify that the logic implemented is indeed this.

Choose the best encoding for a file handle.

This function implements the same logic as chooseBestEncPure, but in a way that is more optimal in terms of IO queries. In particular:

1. It receives both a handle and its current encoding, because the calling function will, most likely, need to know the current encoding (e.g. to be able to restore it), so we avoid repeating the query.

2. It first checks for the cases where it doesn't care whether the device is a terminal or not, so the query will be made only if really necessary.