"Simon Marlow"
I hope you weren't including GHC in "most compilers" :-) GHC's implementation of mutually recursive modules is conformant with Haskell 98 (see Section 5.7).
It depends on how you interpret this phrase: "may require that imported modules contain additional information" If you count writing a separate .hs-boot file as being the additional information that is somehow "contained" within the module, then I suppose nhc98 is also conformant with the report. The only difference with nhc98 is that you hand-write a .hi file, rather than a .hs-boot file, but the syntax is very similar to plain Haskell. Also, the hand-written .hi file will be replaced with a machine-generated one after compilation, but that is only a problem if the user inadvertently removes all the .hi files. Regards, Malcolm