Embedding Lisp in C++ – A Recipe

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2021-06-08 06:30:12

Lisp may be said to be simultaneously the most common and near enough most uncommon programming language in the world. We can quantify this. Head over to the Tiobe Index of Programming Languages at http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html .

This of course proves nothing besides popularity. A single lens reflex camera, while infinitely more useful for photography, is still less common than cameras built into smart phones.

When I say Lisp is more common than any other programming language, I mean this: No matter if you are programming in Python or C++, Lisp is invariably what holds up the scaffolding behind the scenes. This is true because Lisp is the Lingua Franca of your compiler. Your compiler does not work with the syntax that you see, semicolons in C++ or significant white space in Python. Rather it discards this at the earliest possible moment and converts your code into an abstract syntax tree or AST. That AST is composed of lists of lists containing statements and expressions. Your compiler prefers this format for working with your code because it needs a format that is suitable for representing code as data, one of the core tenets of Lisp. It needs this because it must be able to both transform and optimise your code. For example, it may want to elide, re-arrange, or parallelize. Yet it must reason about the equivalence of the transformations it makes. In other words, your compiler needs to “calculates code,” precisely what Lisp is great at by way of its homoiconic syntax. So your compiler borrows this concept to build one or more Intermediate Representations (IR) using abstract syntax trees. Being a List Processor and working with Abstract Syntax Trees, your compiler may essentially be regarded as a Lisp engine. We can extract this intermediate representation from compilers like GCC and CLANG.

Immediately clear is the canonically correct Lisp indentation and code layout. This format is actually very illuminating because it makes it obvious, for example, where type casts have been inferred by the compiler. Similarly, we may elicit our AST from GCC as shown below. We use “g++ -fdump-rtl-dfinish hello fun.cpp“.