It is called a compiler even it outputs code in another language, transpiler is some neologism from JavaScript developers without a background in compiler design.
I'd defend the word 'transpiler' and I did my masters and PhD on language implementation, and that's where I work professionally now, so I'm not ignorant of compilers.
I think it implies a translation from one high level language to another (not that 'high level' is well defined either), with only desugaring and maybe type-checking - no real lowering or optimisations. That's a useful subset of compilers, so can have its own word I believe.
I think a transpiler is a compiler that does a high-level to high-level translation with only a few simple transformations and almost no optimisations and produces relatively readable output.
If a C or Eiffel compiler meets that definition then yes. I'd still call it a compiler, but I would also call it a transpiler.
If you're offended by the term transpiler because all transpilers are also compilers, then I don't know why you wouldn't also be offended by the term compiler because all compilers are also 'programs', so lets just call everything a 'program'.
Why are people giving this word so much flak? Yes it's a neologism, but it's a useful one. A decompiler is a compiler to, it just goes from a low level language to a high level one. Do you suggest that we stop using the word decompiler as well?
> It is called a compiler even it outputs code in another language
Indeed and most compilers do exactly that. Cant help getting a little annoyed whenever I hear this 'transpiler' word. I guess the ship has sailed, oh well !
From what I've noticed, these "transpilers" output code that is readable (the code itself is written as though a human wrote it) where as "compilers" output code that has been optimized and show effects of name mangling in the code itself, etc. Just an observation.
I think it makes sense to use a different term for this "compiler"-esque behavior. For example, I might edit the output of CoffeeScript generated Javascript whereas I wouldn't know how to modify the output of gcc.
This is even better highlighted by something like the GHC compiler, which compiles Haskell to (ultimately) machine code, because it does this in several steps:
1. As a first step, it "transpiles" Haskell to the very similar high-level intermediate language called Core. I could read and write programs in Core without too much effort because it's still a very high-level language.
2. Then it "transpiles" the Core code to the not significantly lower level STG source code – human readable, but starts dealing in a little lower-level stuff and contains much fewer bells and whistles. I'd prefer not to work in STG, but I totally could if I had to.
3. After this, it again "transpiles" the STG source code to the C-- (C-minus-minus) intermediate language, which is decidedly low-level (slightly lower level than regular C code), but there's still a clear and relatively simple translation between STG and C--.
4. As a final step, it "transpiles" the C-- source code to assembly (a closer match than you may think, coming from C), which can be assembled into machine code.
At no point in this pipeline is there a significant magic translation being made – despite the fact that Haskell is probably one of the most high-level languages we know and machine code is as low-level as you get.
People believe some languages are "higher level" than others and call a tool that translates programs written in A to programs written in B a compiler if A is clearly higher level than B and a transpiler if B is of a level higher than or roughly equal to A.
You can certainly think this distinction doesn't merit using a different word, but you shouldn't think that people who use the word "transpile" use it as a synonym for "compiler".
An optimizing cispiler sounds really interesting. Something that applies all the optimizations that the compiler would do when translating your C to machine code but showing you the results in C. Of course not all optimizations are representable in C (calling conventions for instance) but many are.
There are several examples of this in the Java world, such as proguard. They're mostly centered around code obfuscation, but they do some useful optimizations as well.
