I expected "read"
to work directly on a Data.Text because Data.Text has
an instance of the Read class, but it seems to fail:
An instance of Read means that read can *produce* a Text, not that it can consume one. read always reads from a String, as you can see from its type:
Prelude Data.Text> :t read
read :: Read a => String -> a
(Note that it is the result type a in the context for Read.)
See the Data.Text.Read module (part of the text package you already have installed) for how to do similar things with a Text as a source.