Hi Günther The finally tagless style is an implementation of the TypeCase pattern (Bruno C. d. S. Oliveira and Jeremy Gibbons): http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/typecase.pdf TypeCase can be implemented via GADTs or type classes - typed printf in section 4.2 of the paper is shown in both versions. The authors of the "Finally Tagless" note in the long version of their paper that the GADT TypeCase had some problems with non-exhaustive pattern matching (last paragraph, page 14) - if I'm understanding it correctly, in order to be total, the interpretation function stills needs to introduce pattern matching clauses in some circumstances for values that the GADT actually prevents occurring. Plain old data types can easily be interpreted by multiple evaluators - the benefit of TypeCase is probably when you want some degree of extensibility, see this message for instance: http://www.haskell.org/pipermail/haskell-cafe/2008-July/045028.html Best wishes Stephen