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Recent work showed that compiling functional programs to use dense, serialized memory representations for recursive algebraic datatypes can yield significant constant-factor speedups for sequential programs. But serializing data in a maximally dense format consequently serializes the processing of that data, yielding a tension between density and parallelism. This paper shows that a disciplined, practical compromise is possible. We present Parallel Gibbon, a compiler that obtains the benefits of dense data formats and parallelism. We formalize the semantics of the parallel location calculus underpinning this novel implementation strategy, and show that it is type-safe. Parallel Gibbon exceeds the parallel performance of existing compilers for purely functional programs that use recursive algebraic datatypes, including, notably, abstract-syntax-tree traversals as in compilers. 

Solar eclipses are magnificent natural phenomena during which the sun is obscured by the moon. Besides the unique opportunity of studying the solar corona and immediate vicinity of the sun, an eclipse also leads to a darkened daytime sky with sunset colors and many other fascinating phenomena. Here we focus on how the daytime horizontal visual range changed during the duration of the solar eclipse of 21 August 2017, observed from Rexburg, Idaho, USA. Close to totality the eastern horizon for a short time period showed the contours of the Grand Teton Mountains from distances between about 80 km to 90 km. We show and discuss photographic images that show the visual range during totality being significantly extended beyond the visual range in most of the partial phase before and after totality, which was below 80 km when the mountains could not be seen by the naked eye. This phenomenon of an extended visual range can be explained in terms of a simple model for the daytime visual range. This model, which will be explained in this presentation, nicely reproduces the observations and also predicts other phenomena; for example, it predicts that similar phenomena may be observed if part of the line of sight close to the observer is in deep shade of a thick cloud cover. The presentation will tie these observations and their explanation to the teaching of optical scattering and atmospheric optics. 

Mirages, rainbows, halos, glories, and coronas are well-known atmospheric optics phenomena that can be used as examples when teaching the well-understood underlying optical principles of refraction, reflection, dispersion, diffraction, and scattering. These beautiful natural phenomena can generate interest when used for teaching optics because they usually are easily observable with the naked eye. The invention of Si-sensor-based digital photography, however, also offers easy investigations in spectral regions adjacent to the visible range. We report and discuss observations of natural phenomena with a modified DSLR camera operating in the near infrared spectral range above 800 nm. This investigation may be particularly useful in photonics education, as the discussion of photon detectors in certain wavelength ranges can be combined with atmospheric optics, which always attracts interest in students.