Export lists in modules

[Ben Rudiak-Gould]

Malcolm Wallace wrote:
> An explicit interface would be useful for many purposes besides
> machine-checked documentation.  For instance, it could be used to
> eliminate the hs-boot or hi-boot files used by some compilers when
> dealing with recursive modules.

Why *does* ghc require hs-boot files? What can be gleaned from an hs-boot 
file that couldn't be expressed in the corresponding hs file? For example, 
why doesn't ghc simply require that at least one module in a recursive group 
contain an explicit export list mentioning only explicitly typed symbols?

-- Ben