Merge pull request #380 from mhvk/cycle4-funding

Cycle 4 funding: uncertainties, masks, quantities, other types of arrays

finance/proposal-calls/cycle4/mhvk-uncertainty-masked-units-development.md

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+### Title
+
+Support for Uncertainties, Masks, Improved Quantities, Other Types of Arrays
+
+### Project Team
+
+Marten van Kerkwijk
+
+### Project Description
+
+I request partial buy-out from my professorship at UofT to be able to work one
+day a week on projects that are too large for the time I can currently commit
+for astropy.  Specifically, I propose,
+
+- To ensure Masked and Distribution can be used in all the main astropy classes
+  (Time, Representation, Frame, and SkyCoord).
+- Use the new numpy dtype machinery to deal with units, thus speeding up units
+  conversions.
+- Facilitate Quantity becoming a container class that can handle not just
+  ndarray but any type of array, i.e., also dask, jax, etc.
+- Extend the same machinery to Masked and Distribution so that all main astropy
+  classes can use arbitrary array classes.
+- Finish my implementation of a Variable class that tracks uncertainties and
+  their correlations analytically (based on the uncertainties package).
+
+### Project / Work
+
+Currently, I spent about a day per week on astropy core, in reviews, bug
+fixes, and development.  While I have managed to use extra time for fairly
+large developments (Quantity historically and Masked and Uncertainty more
+recently, with also fairly major contributions to Time, Table, Representation
+and numpy), it has been difficult to find enough time to actually wrap up
+larger projects (at least outside sabbaticals).
+
+In particular, while I found time to enable the use of Masked with Quantity
+and Time, the logical extension to coordinates is still missing.  Similarly,
+Distribution now works well with Quantity, but not with Time and the various
+classes underlying coordinates.  Solving this will allow masks and (implicit)
+error propagation on all astropy core classes.  Furthermore, an
+[PR](https://github.com/astropy/astropy/pull/3715) to introduce a Variable
+class that tracks uncertainties and covariances (based on the [uncertainties
+package](https://pythonhosted.org/uncertainties/), but extended it to deal
+natively with arrays), has been stalled for almost a decade.  All these
+require focussed time to finish the implementation, writing tests, documenting
+proper usage, etc.
+
+An exciting development at numpy has been the new dtype machinery, which
+allows much easier design of user data types.  So far, the main application
+has been a new StringDType, which allows having an array of variable-length
+unicode strings (I have been a major reviewer of this, partially to get
+familiar with the new machinery; it may be useful for astropy too).
+
+One of the explicit use cases for the dtype redesign was to support units
+(which can be seen as descriptions of how to interpret the data, with
+converting to another unit similar to casting to another data type).  This
+seems one of the easiest ways to reduce the intricate dependencies on ndarray
+and remove many of the overrides of its methods.  Our unit system itself is
+nicely separated out, which should facilitate using it for a new unit-carrying
+dtype.  Benefits of using this include speed-up (as much more will be done in
+C), and removal of quite tricky code to deal with, e.g., structured data
+types.  I also have some hope of separating out the units/quantity code from
+astropy, so that it can be used more generally.
+
+Using unit-carrying data types should also make it easier (though is not
+required) for Quantity to support other array classes (dask, jax, etc.), as
+proposed in [APE 25](https://github.com/astropy/astropy-APEs/pull/91). This
+will help deal with larger data sets (dask) or use GPU acceleration(jax).
+The [APE 25 report](https://github.com/nstarman/astropy-APEs/blob/units-quantity-2.0/APE25/report.pdf)
+lays out in detail how this could work.  My proposal here is to implement it,
+write proper tests, ensure there are no performance regressions, and of course
+document it all.  A nice benefit of the approach laid out in APE 25 is that it
+will be very easy to extend it to Masked and Distribution (and possibly
+Variable), as those basically are already the type of container classes that
+APE 25 envisions.
+
+A nice things of Quantity being able to use other array types than ndarray is
+that this will nearly automatically extend to coordinates (since those use
+quantities almost exclusively; I foresee little more work than adjusting
+tests!).  Time will be slightly more work, as it works directly with ndarray,
+but also here the path is straightforward: I can just follow my earlier work
+on ensuring Time can work with Masked.
+
+I should perhaps add that the different projects can be separated relatively
+easily, and do not have a very obvious order.  Hence, I can give priority to
+whatever is deemed most important.
+
+### Approximate Budget
+
+I request funding to replace salary equivalent to one day a week, reducing my
+regular employment at the University of Toronto correspondingly.  At a
+standard rate of USD 150/hour (which happens to be roughly my current salary)
+for 8 hours per week and 45 weeks, this corresponds to USD $54000 per year.
+
+Note: I have confirmed with my Chair that a reduction, including of teaching,
+is possible in principle, but am still in the process of finding out how this
+would work in practice.
+
+### Period of Performance
+
+Ideally, this would be for three years (if funding allows and my university
+agrees), but the projects are sufficiently separable that a shorter term is
+useful too (but not fewer hours per week, since the goal is to have full days
+for astropy development only).