Hi Henning,
I wonder whether it is a typical mistake of beginners to write 'return' within a do-block (that is, not at the end) and if it is possible to avoid this mistake by clever typing.
There are legitimate uses of return inside a do, see: http://www.cs.york.ac.uk/fp/darcs/catch/catch_1/Analyse/Fix.hs The code has a bit like: x <- return $ apply x addReq (\y -> y{requiredBy = Set.insert k (requiredBy y)}) x <- return $ apply x delReq (\y -> y{requiredBy = Set.delete k (requiredBy y)}) i.e. modify x, which you can't do with let's because of the scoping (they are really let-rec's). I often use return to do binding and name shadowing in do blocks. Having said that, when I was learning Haskell, I didn't know you could put let in a do block, and did use the return for all variables! Thanks Neil