When someone writes a new programming language, what do they write it IN?

Question

Please excuse my ignorance. I\'m dabbling in PHP and getting my feet wet browsing SO, and feel compelled to ask a question that I\'ve been wondering about for years:

Answer 1

"Writing a new programming language" technically doesn't involve any code. It's just coming up with a specification for what your language looks like and how it works. Once you have an idea of what your language is like, you can write translators and interpreters to actually make your language "work".

A translator inputs a program in one language and outputs an equivalent program in another language. An interpreter inputs a program in some language and runs it.

For example, a C compiler typically translates C source code (the input language) to an assembly language program (the output language). The assembler then takes the assembly language program and produces machine language. Once you have your output, you don't need the translators to run your program. Since you now have a machine language program, the CPU acts as the interpreter.

Many languages are implemented differently. For example, javac is a translator that converts Java source code to JVM bytecode. The JVM is an interpreter [1] that runs Java bytecode. After you run javac and get bytecode, you don't need javac anymore. However, whenever you want to run your program, you'll need the JVM.

The fact that translators don't need to be kept around to run a program is what makes it possible to "bootstrap" your language without having it end up running "on top of" layers and layers of other languages.

[1] Most JVMs do translation behind the scenes, but they're not really translators in that the interface to the JVM is not "input language -> output language".