From the feedback I received so far I learned that it would probably be a good idea to make porting Haskell code to Frege as easy as possible. Which means, not harder than unavoidable.
One of the maybe most annoying differences on the lexical level has been the Java-ish comment syntax. This has been fixed today, and from now on Frege recognizes only Haskell style comments anymore.
Yes, this is a backward-incompatible change. I did it under the assumption that there is not much Frege code around there yet.
For those of you who have already written lots of well-documented Frege code there is a tool to convert old comments to new ones in files specified on the command line. Conversions may be incomplete if one has identifiers with an odd number of apostrophes at the end, as I had to note during conversion of the compiler sources.