A compiler is just a normal program that takes a string and outputs a different string. The input string is lines of code, and the output string is different lines of code. Typically the output string is code in a lower level language (i.e. closer to machine code/pure numbers). An example of a compiler you may be using is Babel, which translates Javascript from newer to older versions. More accurately babel is transpiler since both input and output are Javascript, but that's just a type of compiler.
Interpreters are like compilers, but they translate lines of code one at a time on the fly. Any langauge can be either interpreted or compiled, but most languages are designed with one or the other in mind. Typically an interpreted langauge has a running program that will run your code as as its received. A very common way for this to happen is through a REPL (read-eval-print loop). An example of a REPL is the Javascript console in your web browser. Javascript is designed to be interpreted by web browsers rather than compiled. We're building a compiler, but the basic steps are the same for interpreters - they just happen at a different speed.
Compilers can get very complex, but at their core they usually have 4 basic steps:
- Lexing - break the input code into meaningful chunks (or "words")
- Parsing - convert the words into a data structure
- Optimization - modify the data structure so it will generate efficient output
- Translation - use the data structure to construct output code
For this demo, we'll be implementing a compiler for a very simple langauge that can only express addition. It will take an input string that consists of numbers and '+', and output a line of Lisp code. Each the integers and pluses must be separated by a single space. I chose Lisp as the output because it's my favorite langauge, but also represents math a bit differently, so there'll actually be a difference between input and output.
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