[MLton] mlbs and soft and hard links

[Stephen Weeks]

> I don't agree that viewing things as a map from absolute paths to
> file contents is simpler.  For one thing, there is no file system
> that can check this (the file contents) short of actually reading it
> and comparing it.

Agreed, but this seems OK (or even good) to me.  I was speaking of
simplicity from a semantics perspective, which closely corresponds to
simplicity from the perspective of a user of the system.

> Also rebuilding the cached state on demand has the disadvantage that
> the cache must be located in some place where the current user can
> write.

True.  But this seems necessary for caching more than the basis
library, which is essential to get a decent speedup for type checking
large programs (like MLton).