Re: [Haskell-cafe] Word rigid in "`a' is a rigid type variable..."

Can you recommend any resources that helped you in better understanding?

If you really like type theory, you may want to take at look at Types and Programming Languages by Benjamin C. Pierce. It goes from very simple concepts and builds up, getting into Typed Lambda Calculus and type systems for O.O. languages, for example. http://www.cis.upenn.edu/~bcpierce/tapl/