per-machine maximum number of parallel jobs on a remote machine.

  in parallel.  Hooks are more efficient: locks on output paths are
  only acquired when the hook says that it is willing to accept a
  build job.  Hooks now work in two phases.  First, they should first
  tell Nix whether they are willing to accept a job.  Nix guarantuees
  that no two hooks will ever be in the first phase at the same time
  (this simplifies the implementation of hooks, since they don't have
  to perform locking (?)).  Second, if they accept a job, they are
  then responsible for building it (on the remote system), and copying
  the result back.  These can be run in parallel with other hooks and
  locally executed jobs.

  The implementation is a bit messy right now, though.  

* The directory `distributed' shows a (hacky) example of a hook that
  distributes build jobs over a set of machines listed in a
  configuration file.
  

  no limit).
* Add missing file to distribution.

  distributing a build action to another machine.  In particular, the
  paths in the input closures, the output paths, and successor mapping
  for sub-derivations.

  parallel as possible (similar to GNU Make's `-j' switch).  This is
  useful on SMP systems, but it is especially useful for doing builds
  on multiple machines.  The idea is that a large derivation is
  initiated on one master machine, which then distributes
  sub-derivations to any number of slave machines.  This should not
  happen synchronously or in lock-step, so the master must be capable
  of dealing with multiple parallel build jobs.  We now have the
  infrastructure to support this.

  TODO: substitutes are currently broken.

* Ignore EINTR in reads and writes.

  possible.

  This test fails right now because this hasn't been implemented right
  now.  Yes, I'm doing Test-Driven Development! ;-)

* A test to verify that locking of output paths (caused by concurrent
  invocations of Nix) works correctly.

  environment variables.
* Started adding some automatic tests.
* Do a `make check' when building RPMs.

* Documented release procedures.

  is specified.

  Nix expressions.

  To subscribe to a channel (needs to be done only once):

    nix-channel --add \
      http://catamaran.labs.cs.uu.nl/dist/nix/channels/nixpkgs-unstable

  This just adds the given URL to ~/.nix-channels (which can also be
  edited manually).

  To update from all channels:

    nix-channel --update

  This fetches the latest expressions and pulls cache manifests.  The
  default Nix expression (~/.nix-defexpr) is made to point to the
  conjunction of the expressions downloaded from all channels.

  So to update all installed derivations in the current user
  environment:

    nix-channel --update
    nix-env --upgrade '*'

  If you are really courageous, you can put this in a cronjob or
  something.

  You can subscribe to multiple channels.  It is not entirely clear
  what happens when there are name clashes between derivations from
  different channels.  From nix-env/main.cc it appears that the one
  with the lowest (highest?) hash will be used, which is pretty
  meaningless.

  unless `--disable-init-state' is passed to configure.

  an expression.

  environment variable.  This is useful for passing authentication
  information (it won't show up in `ps').  Hacky - nix-push should
  abstract over the use of Curl.

  efficient by creating only a single symlink to entire directory
  trees unless a collission occurs.

* Made the dependencies on bzip2 and the shell explicit.

  print a nice backtrace of the stack, rather than vomiting a gigantic
  (and useless) aterm on the screen.  Example:

    error: while evaluating file `.../pkgs/system/test.nix':
    while evaluating attribute `subversion' at `.../pkgs/system/all-packages-generic.nix', line 533:
    while evaluating function at `.../pkgs/applications/version-management/subversion/default.nix', line 1:
    assertion failed at `.../pkgs/applications/version-management/subversion/default.nix', line 13

  Since the Nix expression language is lazy, the trace may be
  misleading.  The purpose is to provide a hint as to the location of
  the problem.    
  

  binding.

  instead of `derivation' triggered a huge slowdown in the Nix
  expression evaluator.  Total execution time of `nix-env -qa' went up
  by a factor of 60 or so.

  This scalability problem was caused by expressions such as

    (x: y: ... x ...) a b

  where `a' is a large term (say, the one in
  `all-packages-generic.nix').  Then the first beta-reduction would
  produce

    (y: ... a ...) b

  by substituting `a' for `x'.  The second beta-reduction would then
  substitute `b' for `y' into the body `... a ...', which is a large
  term due to `a', and thus causes a large traversal to be performed
  by substitute() in the second reduction.  This is however entirely
  redundant, since `a' cannot contain free variables (since we never
  substitute below a weak head normal form).

  The solution is to wrap substituted terms into a `Closed'
  constructor, i.e.,

    subst(subs, Var(x)) = Closed(e) iff subs[x] = e

  have substitution not descent into closed terms,

    subst(subs, Closed(x)) = Closed(x)

  and otherwise ignore them for evaluation,

    eval(Closed(x)) = eval(x).

* Fix a typo that caused incorrect substitutions to be performed in
  simple lambdas, e.g., `(x: x: x) a' would reduce to `(x: a)'.