Transitive const sucks

[Janice Caron]

Your problem generalises to anything "streamy"

Going way back to the old C way of doing things
 FILE * f;
 fprintf(f, fmt, stuff);

You'd expect that to work, even if you copied from from inside a const
struct, right? But then the type of f would have to change to

 const FILE * f;

which is C-speak for "f is a const pointer to mutable FILE". Now throwing in
transitivity would stop it all working.

Moving forward in time to the modern era of objects, in general, you would
want:

 stream.write(x);

to work even if the variable "stream" was const. (That is, if the
/reference/ was const, not the data which is pointed to by the reference).
The stack variable can be const, but the heap data needs to be mutable.

I don't have a solution, except to agree that "head const" does seem to be
required after all.
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