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I understand that, at the time when TeX was devised, no single standard for floating point calculations was available. But, these days there is IEEE 754. Why doesn't any TeX variant support it?

Granted, there's LuaTeX, but IEEE 754 was popular long before that, so the question is justified.

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Backward compatibility. – topskip Dec 5 '11 at 12:43
Personally, I would already be satisfied with 64bit integer arithmetic. – Martin Scharrer Dec 5 '11 at 12:53
@Seamus: All plots in my thesis are pgfplots-powered. That uses a lot floating point arithmetic. – Stefan Majewsky Dec 5 '11 at 13:19
When you consider the thinness and preciseness of lines a printer is capable of, coupled to the absorptive properties of paper diffusing the ink (or blurring of toner prior to laser treatment etc.), not to mention the resolution of the human eye, you should be satisfied with pgfplots and 64bit integers at @MartinScharrer says. ;) – qubyte Dec 5 '11 at 15:12
Contrary to the preceding comments, I can think of situations in which floating-point numbers would come in handy. See tex.stackexchange.com/questions/112599/…, for instance. Note that Bruno Le Floch and the rest of the LaTeX3 team have plans to make LaTeX3 compliant with IEEE-754-2008. – Jubobs May 31 '13 at 22:48

It boils down to killing the holy cow of backward compatibility, best explained by Donald Knuth himself in the video on The importance of stability for TeX.

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I think not only that, it is also not a trivial task (seeing the amount of work to add fp calculations to metapost) for no apparent gain. – Khaled Hosny Dec 5 '11 at 13:07
How does adding new features kill backwards compatibility? The only issue I see is name clashes. – Stefan Majewsky Dec 5 '11 at 13:18
@KhaledHosny High accuracy was not needed for Metafont, because it produces bitmaps. For Metapost it's different. – egreg Dec 5 '11 at 13:29
@Stefan true, my thought was that one would replaced the other. Is creeping featurism an issue? – uli Dec 5 '11 at 13:32
@egreg: "no apparent gain" was in reference to TeX not MetaPost. – Khaled Hosny Dec 5 '11 at 15:32

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