I strongly agree with @jpjacobs’s points. Lua is an excellent choice for embedding, unless there’s something very specific about lisp that you need (for instance, if your data maps particularly well to cons-cells).
I’ve used lisp for many many years, BTW, and I quite like lisp syntax, but these days I’d generally pick Lua. While I like the lisp language, I’ve yet to find a lisp implementation that captures the wonderful balance of features/smallness/usability for embedded use the way Lua does.
Lua:
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Is very small, both source and binary, an order of magnitude or more smaller than many more popular languages (Python etc). Because the Lua source code is so small and simple, it’s perfectly reasonable to just include the entire Lua implementation in your source tree, if you want to avoid adding an external dependency.
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Is very fast. The Lua interpreter is much faster than most scripting languages (again, an order of magnitude is not uncommon), and LuaJIT2 is a very good JIT compiler for some popular CPU architectures (x86, arm, mips, ppc). Using LuaJIT can often speed things up by another order of magnitude, and in many cases, the result approaches the speed of C. LuaJIT is also a “drop-in” replacement for standard Lua 5.1: no application or user code changes are required to use it.
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Has LPEG. LPEG is a “Parsing Expression Grammar” library for Lua, which allows very easy, powerful, and fast parsing, suitable for both large and small tasks; it’s a great replacement for yacc/lex/hairy-regexps. [I wrote a parser using LPEG and LuaJIT, which is much faster than the yacc/lex parser I was trying emulate, and was very easy and straight-forward to create.] LPEG is an add-on package for Lua, but is well-worth getting (it’s one source file).
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Has a great C-interface, which makes it a pleasure to call Lua from C, or call C from Lua. For interfacing large/complex C++ libraries, one can use SWIG, or any one of a number of interface generators (one can also just use Lua’s simple C interface with C++ of course).
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Has liberal licensing (“BSD-like”), which means Lua can be embedded in proprietary projects if you wish, and is GPL-compatible for FOSS projects.
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Is very, very elegant. It’s not lisp, in that it’s not based around cons-cells, but it shows clear influences from languages like scheme, with a straight-forward and attractive syntax. Like scheme (at least in it’s earlier incarnations), it tends towards “minimal” but does a good job of balancing that with usability. For somebody with a lisp background (like me!), a lot about Lua will seem familiar, and “make sense”, despite the differences.
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Is very flexible, and such features as metatables allow easily integrating domain-specific types and operations.
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Has a simple, attractive, and approachable syntax. This might not be such an advantage over lisp for existing lisp users, but might be relevant if you intend to have end-users write scripts.
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Is designed for embedding, and besides its small size and fast speed, has various features such as an incremental GC that make using a scripting language more viable in such contexts.
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Has a long history, and responsible and professional developers, who have shown good judgment in how they’ve evolved the language over the last 2 decades.
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Has a vibrant and friendly user-community.