Before starting to write a parser for the Calc language, let's have a look at the most popular parsing techniques used for building both interpreters and compilers. This is needed to understand the Nom library, which uses one of these techniques.
Learning about compiler-compilers and parser combinators
To obtain an extremely fast and flexible parser, you need to build it from scratch. But for decades, an easier approach was used to build parsers by using tools named compiler-compilers or compiler generators: programs that generate compilers. These programs get input as a decorated specification of the syntax and generate the source code of a parser for such a syntax. These generated source code must then be compiled, together with other source files, to get an executable compiler.
This traditional approach is now somewhat out of fashion and another one has emerged, named parser combinator. A parser combinator is a set of functions that allow several parsers...