Table of Contents
eLua features
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Latest release of Lua is 5.2. eLua is based on Lua 5.1.4, and this fact is not widely publicized. (ref: https://github.com/elua/elua/blob/master/src/lua/lua.h) uPy is based on the latest Python release, 3.4 (Note: over time, uPy will certainly linger with updates, too).
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eLua, with LTR patch, boasts only 5.42K RAM usage after boot, down from ~25K with unpatched Lua. (ref: http://www.eluaproject.net/doc/v0.9/en_arch_ltr.html). uPy offers ~540 bytes usage after boot (10 times less than patched eLua), and truly can boot and accept commands with 2K heap.
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Just a quote (http://www.eluaproject.net/doc/v0.9/en_elua_egc.html):
The EGC (Emergency Garbage Collector) patch [...], what it does is that it lets you run a garbage collection cycle in Lua in a low memory situation, from inside Lua's memory allocation function (something that the current version of Lua can't do out of the box).
That's unheard of. In uPy, garbage collection exactly runs when memory allocator can't find enough memory, as a native feature. Note that by "current version of Lua" the previous version of Lua is meant (see above). 5.2 appear to have native support for what most people do consider garbage collection in the first place.
Lua language (mis)features.
- Doesn't error out on usage on invalid and non-existent variables, returns legitimate value for such.
- There's only one numeric type: forced float. Inefficient w/o floating-point hardware (eLua works that around by offering integer-only versions, which then cannot support float, because there's only one numeric type). Forced float also leads to problem with bitwise logical operations. Etc, etc.
- Container type is single, associative array (called "table"). (Latest versions of Lua work that around by providing dynamic array-vs-map switching underneath.) Multiple issues due to this, like array indexing starting from arbitrary number rather than zero, "length" of table not always corresponding to number of elements in it, etc., etc.
- Funky OO method syntax, like obj:close().
- No exceptions, instead pseudo-functional workaround, pcall() and friends. (Also goto in Lua 5.2)
- http://www.tecgraf.puc-rio.br/~lhf/ftp/doc/hopl.pdf p.23 "We have resisted user pressure to include other data structures, mainly “real” arrays and tuples, first by being stubborn, but also by providing tables with an efficient implementation and a flexible design." Of course, what Lua developers call efficient implementation is actually inefficient implementation: "In Lua 5.0 we introduced a hybrid representation for tables: every table contains a hash part and an array part, and both parts can be empty. Lua detects whether a table is being used as an array and automatically stores the values associated to integer indices in the array part, instead of adding them to the hash part" (p.12). So, Lua wastes memory to store "two parts", then wastes runtime cycles to "detect" which one to use.
