Unifying substitutes and successors isn't very feasible for now,
since substitutes are only used when no path with a certain is
known. Therefore, a normal form of some expression stored as a
substitute would not be used unless the expression itself was
missing.
hash for which no local expansion is available, Nix can execute a
`substitute' which should produce a path with such a hash.
This is policy-free since Nix does not in any way specify how the
substitute should work, i.e., it's an arbitrary (unnormalised)
fstate expression. For example, `nix-pull' registers substitutes
that fetch Nix archives from the network (through `wget') and unpack
them, but any other method is possible as well. This is an
improvement over the old Nix sharing scheme, which had a policy
(fetching through `wget') built in.
The sharing scheme doesn't work completely yet because successors
from fstate rewriting have to be registered on the receiving side.
Probably the whole successor stuff can be folded up into the
substitute mechanism; this would be a nice simplification.
