practice

DoI: 10.1145/1965724.1965739

Passing
a Language
Through the
Eye of a needle

Article development led by queue.acm.org

Many languages (not necessarily scripting languages) support extending through a foreign function interface (FFI). An FFI is not enough to allow a function in the system language to do all that a function in the script can do. Nevertheless, in practice FFI covers most common needs for extending, such as access to external libraries and system calls. Embedding, on the other hand, is more difficult to support, because it usually demands closer integration between the host program and the script, and FFI alone does not suffice.

In this article we discuss how embeddability can impact the design of a language, and in particular how it impacted the design of Lua from day one. Lua3, 4 is a scripting language with a particularly strong emphasis on embeddability. It has been embedded in a wide range of applications and is a leading language for scripting games. 2

The Eye of a needle

At first sight, the embeddability of a scripting language seems to be a feature of the implementation of its interpreter. Given any interpreter, we can attach an API to it to allow the host program and the script to interact. The design of the language itself, however, has a great influence on the way it can be embedded. Conversely, if you design a language with embeddability in mind, this mind-set will have a great influence on the final language.

The typical host language for most scripting languages is C, and APIs for these languages are therefore mostly composed of functions plus some types and constants. This imposes a natural but narrow restriction on the design of an API for a scripting language: it must offer access to language features through this eye of a needle. Syntactical constructs are particularly difficult to get through. For example, in a scripting language where methods must be written lexically inside their classes, the host language cannot add methods to a class unless the API offers suitable mechanisms.

How the embeddability of Lua
impacted its design.
BY RoBERTo IERuSALIMSCh Y, LuIZ hEnRIquE DE FIGuEIREDo,
AnD WALDEMAR CELES

sCriptinG LAnGuAGes Are an important element in the current landscape of programming languages. A key feature of a scripting language is its ability to integrate with a system language. 7 This integration takes two main forms: extending and embedding. In the first form, you extend the scripting language with libraries and functions written in the system language and write your main program in the scripting language. In the second form, you embed the scripting language in a host program (written in the system language) so that the host can run scripts and call functions defined in the scripts; the main program is the host program. In this setting, the system language is usually called the host language.