How do you write a compiler for a language in that language? [duplicate]

Question

Possible Duplicates:
How can a language's compiler be written in that language?
implementing a compiler in “itself”

I was looking at Rubinius, a Ruby implementation that compiles to bytecode using a compiler written in Ruby. I cannot get my head around this. How do you write a compiler for a language in the language itself? It seems like it would be just text without anything to compile it into an executable that could then compile the future code written in Ruby. I get confused just typing that sentence. Can anyone help explain this?

Answer 1

To simplify: you first write a compiler for the compiler, in a different language. Then, you compile the compiler, and voila!

So, you need some sort of language which already has a compiler - but since there are many such, you can write the Ruby compiler compiler (!) e.g. in C, which will then compile the Ruby compiler, which can then compile Ruby programs, even further versions of itself.

Of course, the original compilers were written in machine code, compiled compilers for assembly, which in turn compiled compilers for e.g. C or Fortran, which compiled compilers for...pretty much everything. Iterative development in action.

The process is called bootstrapping - possibly named after Baron Munchhausen's story in which he pulled himself out of a swamp by his own bootstraps :)

Answer 2

Regarding the bootstrapping of a compiler it's worth reading about this devilishly clever hack.

http://catb.org/jargon/html/B/back-door.html