Learning multiple languages (question for general discussion)

[Paul Rubin]

"Diez B. Roggisch" <deets at nospam.web.de> writes:
> It's a great book - I cetainly owe it the better part of my thesis
> about multi level specification for functional languages. If you want
> to understand type-systems, its a great comprehensive read.

So do I really want to understand type systems?  I mean, I think I
understand Haskell types from reading the manual, but probably not at
the level of that TAPL book.  Am I going to find out anything
important as an implementer?