Thomas Davie
This brings up something that's been niggling me for a while. Why can Hat not deal with defaulting? Surely it could be handled by the first stage of hat-trans being inserting explicit type signatures based on the standard defaulting rules?
We made a deliberate design decision that hat-trans should be entirely syntax-directed - it does not do type inference. There are good reasons for this decision. * In a well-designed language, you would not need type information for the tracing transformation. And indeed, Haskell is so well designed that you do not, except for this one very small (but very annoying) instance. * Implementing type inference is difficult enough that we did not consider the effort worthwhile for the tiny benefit of being able to handle defaulting. * In order to be able to handle all the many experimental type-system extensions accepted by a compiler like ghc, hat-trans would need to replicate ghc's type inference as exactly as possible. Given the number of papers written about this very topic, that would not be an easy task! But in fact, apart from defaulting, it is only necessary to _syntactically_ transform type signatures (where they are used), and the ghc type system can infer the rest. Regards, Malcolm