Text or Bytestring

ByteStrings are mainly useful for binary data, but they are also an efficient way to process text if all you need is the ASCII character set. If you need to handle unicode strings, you need to use Text. However, I must emphasize that neither is a replacement for the other, and they are generally used for different things: while Text represents pure unicode, you still need to encode to and from a binary ByteString representation whenever you e.g. transport text via a socket or a file.

Here is a good article about the basics of unicode, which does a decent job of explaining the relation of unicode code-points (Text) and the encoded binary bytes (ByteString): The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets

You can use the Data.Text.Encoding module to convert between the two datatypes, or Data.Text.Lazy.Encoding if you are using the lazy variants (as you seem to be doing based on your error messages).

You definitely want to be using Data.Text for textual data.

encodeUtf8 is the way to go. This error:

Couldn't match expected type Data.ByteString.Lazy.Internal.ByteString against inferred type Data.ByteString.Internal.ByteString

means that you're supplying a strict bytestring to code which expects a lazy bytestring. Conversion is easy with the fromChunks function:

Data.ByteString.Lazy.fromChunks :: [Data.ByteString.Internal.ByteString] -> ByteString

so all you need to do is add the function fromChunks [myStrictByteString] wherever the lazy bytestring is expected.

Conversion the other way can be accomplished with the dual function toChunks, which takes a lazy bytestring and gives a list of strict chunks.

You may want to ask the maintainers of some packages if they'd be able to provide a text interface instead of, or in addition to, a bytestring interface.

Use a single function cs from the Data.String.Conversions.

It will allow you to convert between String, ByteString and Text (as well as ByteString.Lazy and Text.Lazy), depending on the input and the expected types.

You still have to call it, but no longer to worry about the respective types.

See this answer for usage example.