Explaining dependency resolution failures

docs/err/dep-res-failure.rst

+Error: Dependency Resolution Failure
+####################################
+
+.. note::
+    ``dds`` implements the `Pubgrub`_ dependency resolution algorithm.
+
+.. _Pubgrub: https://github.com/dart-lang/pub/blob/master/doc/solver.md
+
+If you receive this error, it indicates that the requested dependencies create
+one or more conflicts. ``dds`` will do its best to emit a useful explanation of
+how this conflict was formed that can hopefully be used to find the original
+basis for the conflict.
+
+Every package can have some number of dependencies, which are other packages
+that are required for the dependent package to be used. Beyond just a package
+name, a dependency will also have a *compatible version range*.
+
+
+Resolution Example
+******************
+
+For example, let us suppose a package ``Widgets@4.2.8`` may have a *dependency*
+on ``Gadgets^2.4.4`` and ``Gizmos^3.2.0``.
+
+.. note::
+    The ``@4.2.8`` suffix on ``Widgets`` means that ``4.2.8`` is the *exact*
+    version of ``Widgets``, while the ``^2.4.4`` is a *version range* on
+    ``Gadgets`` which starts at ``2.4.4`` and includes every version until (but
+    not including) ``3.0.0``. ``^3.2.0`` is a version range on ``Gizmos`` that
+    starts at ``3.2.0`` and includes every version until (but not including)
+    ``4.0.0``.
+
+Now let us suppose the following versions of ``Gadgets`` and ``Gizmos`` are
+available:
+
+``Gadgets``:
+    - ``2.4.0``
+    - ``2.4.3``
+    - ``2.4.4``
+    - ``2.5.0``
+    - ``2.6.0``
+    - ``3.1.0``
+
+``Gizmos``:
+    - ``2.1.0``
+    - ``3.2.0``
+    - ``3.5.6``
+    - ``4.5.0``
+
+We can immediately rule out some candidates of ``Gadgets``: for the dependency
+``Gadgets^2.4.4``, ``2.4.0`` and ``2.4.3`` are *too old*, while ``3.1.0`` is
+*too new*. This leaves us with ``2.4.4``, ``2.5.0``, and ``2.6.0``.
+
+We'll first look at ``Gadgets@2.4.4``. We need to recursively solve its
+dependencies. Suppose that it declares a dependency of ``Gizmos^2.1.0``. We
+have already established that we *require* ``Gizmos^3.2.0``, and because
+``^2.1.0`` and ``^3.2.0`` are *disjoint* (they share no common versions) we can
+say that ``Gizmos^3.2.0`` is *incompatible* with our existing partial solution,
+and that its dependent, ``Gadgets@2.4.4`` is *transitively incompatible* with
+the partial solution. Thus, ``Gadgets@2.4.4`` is out of the running.
+
+This doesn't mean we're immediately broken, though. We still have two more
+versions of ``Gadgets`` to inspect. We'll start with the next version in line:
+``Gadgets@2.5.0``. Suppose that it has a dependency on ``Gizmos^3.4.0``. We
+have already established a requirement of ``Gizmos^3.2.0``, so we must find
+a candidate for ``Gizmos`` that satisfies both dependencies. Fortunately, we
+have exactly one: ``Gizmos@3.5.6`` satisfies *both* ``Gizmos^3.2.0`` *and*
+``Gizmos^3.4.0``.
+
+Suppose that ``Gizmos@3.5.6`` has no further dependencies. At this point, we
+have inspected all dependencies and have resolutions for every named package:
+Thus, we have a valid solution of ``Widgets@4.2.8``, ``Gadgets@2.5.0``, and
+``Gizmos@2.6.0``! We didn't even need to inspect ``Gadgets@2.6.0``.
+
+In this case, ``dds`` will not produce an error, and the given package solution
+will be used.
+
+
+Breaking the Solution
+=====================
+
+Now suppose the same case, except that ``Gadgets@2.5.0`` is not available.
+We'll instead move to check ``Gadgets@2.6.0``.
+
+Suppose that ``Gadgets@2.6.0`` has a dependency on ``Gizmos^4.0.6``. While we
+*do* have a candidate thereof, we've already declared a requriement on
+``Gizmos^3.2.0``. Because ``^4.0.6`` and ``^3.2.0`` are disjoint, then there is
+no possible satisfier for both ranges. This means that ``Gizmos^4.0.6`` is
+incompatible in the partial solution, and that ``Gadgets@2.6.0`` is
+transitively incompatible as well. It is no longer a candidate.
+
+We've exhausted the available candidates for ``Gadgets^2.4.4``, so we must now
+conclude that ``Gadgets^2.4.4`` is *also incompatible*. Transitively, this also
+means that ``Widgets@4.2.8`` is incompatible as well.
+
+We've reached a problem, though: ``Widgets@4.2.8`` is our original requirement!
+There is nothing left to invalidate in our partial solution, so we rule that
+our original requirements are *unsatisfiable*.
+
+At this point, ``dds`` will raise the error that *dependency resolution has
+failed*. It will attempt its best to reconstruct the logic that we have used
+above in order to explain what has gone wrong.
+
+
+Fixing the Problem
+******************
+
+There is no strict process for fixing these conflicts.
+
+Fixing a dependency conflict is a manual process. It will require reviewing the
+available versions and underlying reasons that the dependency maintainers have
+chosen their compatibility ranges statements.
+
+Your own dependency statements will often need to be changed, and sometimes
+even code will have to be revised to reach compatibility with newer or older
+dependency versions.