Re: Download Parsec
Tanja Freeze
The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913[4] to simplify astronomers' calculations of astronomical distances from only raw observational data. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.
One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is taken approximately half a year later, when the Earth is on the opposite side of the Sun.[b] The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the distant vertex. Then the distance to the star could be calculated using trigonometry.[7] The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate the 3.5-parsec distance of 61 Cygni.[8]
The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the semimajor axis of the Earth's orbit. Substituting the star's parallax for the one arcsecond angle in the imaginary right triangle, the long leg of the triangle will measure the distance from the Sun to the star. A parsec can be defined as the length of the right triangle side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond.
The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the reciprocal of the parallax angle in arcseconds (i.e.: if the parallax angle is 1 arcsecond, the object is 1 pc from the Sun; if the parallax angle is 0.5 arcseconds, the object is 2 pc away; etc.). No trigonometric functions are required in this relationship because the very small angles involved mean that the approximate solution of the skinny triangle can be applied.
Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He proposed the name astron, but mentioned that Carl Charlier had suggested siriometer and Herbert Hall Turner had proposed parsec.[4] It was Turner's proposal that stuck.
A corollary states that a parsec is also the distance from which a disc that is one au in diameter must be viewed for it to have an angular diameter of one arcsecond (by placing the observer at D and a disc spanning ES).
ESA's Gaia satellite, which launched on 19 December 2013, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Centre, about 8000 pc away in the constellation of Sagittarius.[15]
Distances expressed in parsecs (pc) include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of 1,000 parsecs (3,262 ly) is denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to express distances between parts of a galaxy or within groups of galaxies. So, for example :
To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs[c] (kpc3) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds, and interstellar gas is determined in a similar fashion. To determine the number of galaxies in superclusters, volumes in cubic megaparsecs[c] (Mpc3) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge Botes void is measured in cubic megaparsecs.[20]
The parsec was seemingly used incorrectly as a measurement of time by Han Solo in the first Star Wars film, when he claimed his ship, the Millennium Falcon "made the Kessel Run in less than 12 parsecs". The claim was repeated in The Force Awakens, but this was clarified in Solo: A Star Wars Story, by stating the Millennium Falcon traveled a shorter distance (as opposed to a quicker time) due to a more dangerous route through the Kessel Run, enabled by its speed and maneuverability.[21]
Since iot 37 my parsec service has been failing after reboot. It seems that the config file and the library files keep getting chowned to dnsmasq from parsec. Im not sure whats causing this but if i chown back to parsec everything works properly. What is the proper solution for this?
I'm new to Haskell and Parsec. After reading Chapter 16 Using Parsec of Real World Haskell, a question appeared in my mind: Why and when is Parsec better than other parser generators like Yacc/Bison/Antlr?
My understanding is that Parsec creates a nice DSL of writing parsers and Haskell makes it very easy and expressive. But parsing is such a standard/popular technology that deserves its own language, which outputs to multiple target languages. So when shall we use Parsec instead of, say, generating Haskell code from Bison/Antlr?
This question might go a little beyond technology, and into the realm of industry practice. When writing a parser from scratch, what's the benefit of picking up Haskell/Parsec compared to Bison/Antlr or something similar?
A parser generator reads in a description of a grammar and spits out a parser. It is generally not possible to combine existing grammars into a new grammar, and it is certainly not possible to combine two existing generated parsers into a new parser.
A parser combinator OTOH does nothing but combine existing parsers into new parsers. Usually, a parser combinator library ships with a couple of trivial built-in parsers that can parse the empty string or a single character, and it ships with a set of combinators that take 1 or more parsers and return a new one that, for example, parses the sequence of the original parsers (e.g. you can combine a d parser and an o parser to form a do parser), the alternation of the original parsers (e.g. a 0 parser and a 1 parser to a 01 parser) or parses the original parse multiple times (repetetion).
In Haskell the competition is between Parsec (and other parser combinators) and the parser generator Happy. I'd pick Happy if I already had an LR grammar to work from - parser combinators take grammars in LL form and the translation from LR to LL takes some effort and a combinator parser will usually be significantly slower. If I don't have a grammar I'll use Parsec, it is more flexible (powerful) than Happy and its more fun to work "in Haskell" than generate code with Happy and Alex. If you use Happy for parsing you almost always need to use Alex for lexing.
For industry practice, it would be odd to decide to use Haskell just to get Parsec. For parsing, most of the current crop of languages will have at least a parser generator and probably something more flexible like a port of Parsec or a PEG system.
Ira Baxter's answer to the linked question was spot-on about a parser getting you merely to the foothold of the Himalayas for writing a translator, but being part of a translator is only one of the uses for a parser, so there are still many domains where fairly minimalist systems like ANTLR, Happy and Parsec are satisfactory.
Following on from stephen's answer, I think that one of the most common alternatives to Parsec, if you want to stick with parser combinators, is attoparsec. The main difference is that attoparsec was written with more of a bias towards speed, and makes trade-offs accordingly. For example, Parsec does some book-keeping to try to return helpful error messages if a parse fails, which attoparsec doesn't do to the same extent. Also, I think that attoparsec is specialised to one input stream/token type, whereas Parsec abstracts from the input type so that it can parse streams of type String, ByteString, Text, etc. without problem.
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