[Nix-dev] Dependency Semantics

[Bryce L Nordgren]

Hi list,

I want to suggest something to see what you think.

Nix currently supports a single meaning for its dependency relation, which
essentially can be summed up as "Depends on exact build of". As in, GDAL
<depends on exact build of> GEOS(hash here). This is an enormously
successful model, but it does force you to completely omit real
dependencies (and to trick Nix into not detecting them). For instance, to
avoid recompiling all Java source to bytecode simply because some change
triggers the rebuilding of the java compiler (Note we're not even talking
about upgrading Java here: we're saying, i.e., a sound library was upgraded
so the same JDK was rebuilt.), it has been suggested to NOT depend on the
jdk and to refer to the java interpreter only by an environment variable so
that Nix won't pick up the dependency (e.g., $JAVA_HOME/bin/java).
Likewise, "hash rewriting" has also been suggested as a manual workaround
for those situations where the "Depends on exact build of" semantics are
inappropriate. Certainly "Depends on exact build of" will produce a
workable result given enough time and CPU cycles, but there are situations
where it is extremely wasteful. Consider the following additional
semantics:

* "requires environment" : Requires the presense of the interpreter
software/VM (e.g. bash/python/perl/java/ruby) but does not force the
dependent package to be rebuilt (and stored again) if the interpreter/VM is
replaced. The target of the dependency does not represent a specific build,
but an environment description.
* "implements environment" : Declares that the package is designed to
provide a specific environment. Two implementations of Ruby 1.8.7 could be
the reference implementation of Ruby or Ruby-Enterprise. The target of the
dependency represents an environment description.
* "depends on executables from" : Refers to a Nix source expression instead
of a specific build. The dependency is on a command line utility which has
already been linked. The dependant package is not rebuilt when the target
is. Also: any build instantiated from the target expression should be
considered interchangable. For example: a dependency on the "dot"
executable from GraphVis should not cause Doxygen to be rebuilt if a
rebuild of GraphVis is triggered; conversely, any build of GraphVis
(version X) present on the system should satisfy the requirement.

I understand that there is some resistance to these suggestions because
they appear too similar to the semantics present in other package
management systems (which completely lack "depends on exact build of").
This is not an effort to make Nix more "familiar" or to remove "depends on
exact build of". It's intended to eliminate wasteful and needless
rebuilding (while reducing the number of missed/masked dependencies) by
allowing the packager to correctly express relationships between packages
which are not "depends on exact build of". Additionally, this would allow
"nix-env" "nix-build" and Hydra to correctly process these directives.

Thoughts?

Bryce
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