Hi Niklas,
Do I have to write my own prettyprinter? Do I have to put in explicit parentheses? The latter seems unsatisfactory as my generated AST is unambiguous and bracketing ought to be part of the prettyprinter. The former would be quite a lot of code as there are many cases to consider.
Looking at your example, what you want is brackets to be inserted whenever the right subexpression in an application is non-atomic. That certainly seems reasonable to me. Would you file a ticket for it please? http://trac.haskell.org/haskell-src-exts :-)
I wanted that once, then I realised I was wrong :-) Should you insert brackets everywhere it's ambiguous? What about operators - you don't know the fixities, so can't tell if you should insert a bracket or not. I can't see any way for HSE to implement this feature correctly, and as such, it's really not a good feature to push down in to the pretty printer. In HLint I have a bracketing module, which has served me well. Please take any ideas you need from it - http://community.haskell.org/~ndm/darcs/hlint/src/HSE/Bracket.hs . In particular, given a fully bracketed expression, I can call transformBracket to transform the expression, not caring about brackets, in a way that guarantees the right brackets are put back. There is also needBracket and isAtom which are very useful. If you call descendBi (transformBracket Just) it will automatically bracket your term as much as is necessary. If you don't understand any of the ideas in Bracket then look at Uniplate (http://community.haskell.org/~ndm/uniplate) - it's where a lot of the ideas came from. If you're working with a HSE source tree without using Uniplate (or a competitor) then you're doing it wrong. Thanks, Neil