I understand that a self-hosting compiler is a compiler which can compile the code of the language that it is written in into different language but I don't understand why we would want to do that. What are the benefits (and drawbacks) of a compiler which is self-hosting?

  • $\begingroup$ Are you sure you haven't mixed bootstraping with transpiler? $\endgroup$ – Evil May 1 '16 at 17:11
  • $\begingroup$ I don't think so. A self-hosting compiler takes as input the language that it is written in, right? $\endgroup$ – Haych May 1 '16 at 17:13
  • $\begingroup$ Right, it compiles itself. What about the part of source-to-source compilation? $\endgroup$ – Evil May 1 '16 at 17:27
  • $\begingroup$ What do you mean source-to-source compilation? $\endgroup$ – Haych May 1 '16 at 18:44
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    $\begingroup$ "self-hosting compiler is a compiler which can compile it's source language into another" - this is the unclear part to me. $\endgroup$ – Evil May 1 '16 at 18:45

There's no direct technical benefit for a compiler to be self-hosting. If anything, it makes things more complicated, because you have to have a working compiler before you can compile the compiler — the bootstrapping problem. (Solved either by having another implementation of the same language — often an interpreter — or by keeping around binaries of a previous compiler version.)

Compiler writers often like the language that they're writing a compiler for. This is the primary reason why many compilers are self-hosting.

Having a compiler that compiles itself does have an indirect technical benefit: it's a litmus test. A compiler is a complex program. If the language is comfortable enough to write a program in, that's a sign that it's comfortable enough for a large class of programs. If the compiler can compile itself without bugs, that's a sign that it's in a good enough shape to compile other programs with confidence.

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Think about how the first compiler got written, for the first programming language supported by some platform (probably a C compiler). In other words, think about writing a C compiler for a platform where there is no other compiler available for any other language.

Your only option is to write the C compiler in assembly language. But writing an entire compiler in assembly language would be incredibly painful. You'd much rather write the compiler in a high-level language than in assembly (for the same reason you'd rather write pretty much any program in a high-level language).

If you can write a self-hosting compiler, then you've achieved that. The usual way to construct a self-hosting compiler is through bootstrapping. Wikipedia has articles on both.

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  • $\begingroup$ An additional reason: to extend the "reach" of the language. This was the motivation for making Clojurescript self-compiling, google "Bootstrapped-ClojureScript-FAQ Clojurescript FAQ". $\endgroup$ – user48832 May 1 '16 at 20:21
  • $\begingroup$ I think I am not understanding something obvious here. How have we achieve this? I don't understand the link between the code a compiler is written in and the input code of that compiler. $\endgroup$ – Haych May 2 '16 at 8:37

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