Source-to-source compiler

A source-to-source compiler (S2S compiler), transcompiler or transpiler is a type of compiler that takes the source code of a program written in a programming language as its input and produces the equivalent source code in the same or a different programming language. A source-to-source compiler translates between programming languages that operate at approximately the same level of abstraction, while a traditional compiler translates from a higher level programming language to a lower level programming language. For example, a source-to-source compiler may perform a translation of a program from Python to JavaScript, while a traditional compiler translates from a language like C to assembler or Java to bytecode[1]. An automatic parallelizing compiler will frequently take in a high level language program as an input and then transform the code and annotate it with parallel code annotations (e.g., OpenMP) or language constructs (e.g. Fortran's forall statements).[2]

Another purpose of source-to-source-compiling is translating legacy code to use the next version of the underlying programming language or an API that breaks backward compatibility. It will perform automatic code refactoring which is useful when the programs to refactor are outside the control of the original implementer (for example, converting programs from Python 2 to Python 3, or converting programs from an old API to the new API) or when the size of the program makes it impractical or time consuming to refactor it by hand.

Transcompilers may either keep translated code structure as close to the source code as possible to ease development and debugging of the original source code, or may change the structure of the original code so much that the translated code does not look like the source code.[3] There are also debugging utilities that map the transpiled source code back to the original code; for example, the JavaScript Source Map[4] standard allows mapping of the JavaScript code executed by a web browser back to the original source when the JavaScript code was, for example, minified or produced by a transpiled-to-JavaScript language.[5]

Examples of transcompiled languages include Closure Compiler, CoffeeScript, Dart, Haxe, TypeScript and Emscripten.[6]


One of the earliest programs of this kind was Digital Research's XLT86 in 1981, a program written by Gary Kildall, which translated .ASM source code for the Intel 8080 processor into .A86 source code for the Intel 8086. Using global data flow analysis on 8080 register usage, the translator would also optimize the output for code size and take care of calling conventions, so that CP/M-80 and MP/M-80 programs could be ported to the CP/M-86 and MP/M-86 platforms automatically. XLT86 itself was written in PL/I-80 and was available for CP/M-80 platforms as well as for DEC VMS (for VAX 11/750 or 11/780).[7]

A similar, but much less sophisticated program was TRANS.COM, written by Tim Paterson in 1980 as part of 86-DOS. It could translate some Z80 assembly source code into .ASM source code for the 8086, but supported only a subset of opcodes, registers and modes, often still requiring significant manual correction and rework afterwards. Also it did not carry out any register and jump optimizations.[8][9]

Programming language implementations

The first implementations of some programming languages started as transcompilers, and the default implementation for some of those languages are still transcompilers. In addition to the table below, a CoffeeScript maintainer provides a list of languages that compile to JavaScript.[10]

List of transpilers[11]
Name Source language Target language
Cfront C++ C
HipHop for PHP (HPHPc) PHP C++
Babel JavaScript ES6 JavaScript ES5
ClojureScript Clojure JavaScript
JSweet Java TypeScript
Swiftify Objective-C Swift
J2ObjC Java Objective-C

Porting a codebase

When developers want to switch to a different language while retaining most of an existing codebase, it might be better to use a transcompiler compared to rewriting the whole software by hand. Depending on the quality of the transcompiler, the code may or may not need manual intervention in order to work properly. This is different from "transcompiled languages" where the specifications demand that the output source code always works without modification. All transpilers used to port a codebase will expect manual adjustment of the output source code if there is a need to achieve maximum code quality in terms of readability and platform convention.

Transcompiler pipelines

A transcompiler pipeline is what results from recursive transcompiling. By stringing together multiple layers of tech, with a transcompile step between each layer, technology can be repeatedly transformed, effectively creating a distributed language independent specification.

XSLT is a general purpose transform tool which can be used between many different technologies, to create such a derivative code pipeline.

See also


  1. "Transpiler". Retrieved 2019-06-22.
  2. "Types of compilers". 1997–2005. Retrieved 28 October 2010.
  3. Fowler, Martin (February 12, 2013). "Transparent Compilation". Retrieved February 13, 2013.
  4. Lenz, Fitzgerald (11 February 2011). "Source Map Revision 3 Proposal".
  5. Seddon, Ryan (21 March 2012). "Introduction to JavaScript Source Maps". Retrieved 21 January 2015.
  6. Epic Games; Mozilla. "HTML5 Epic Citadel".
  7. Digital Research (1981). XLT86 - 8080 to 8086 Assembly Language Translator - User's Guide (PDF). Pacific Grove, CA, USA: Digital Research Inc. Archived (PDF) from the original on 2016-11-18. Retrieved 2016-11-18.
  8. Seattle Computer Products (1980): 86-DOS - Disk Operating System for the 8086. User's manual, version 0.3 - Preliminary. Seattle Computer Products, Seattle ().
  9. Paterson, Tim (2013-12-19) [1982]. "Microsoft DOS V1.1 and V2.0: Z80 to 8086 Translator version 2.21 /msdos/v11source/TRANS.ASM". Computer History Museum, Microsoft. Retrieved 2014-03-25. (NB. While the publishers claim this would be MS-DOS 1.1 and 2.0, it actually is SCP MS-DOS 1.25 and TeleVideo PC DOS 2.11.)
  10. "List of languages that compile to JS". Retrieved 2018-03-11.
  11. "Transpiler". Retrieved 2019-01-08.
  12. "C->Go translator". Retrieved 11 January 2018.
  13. "Go 1.5 Release Notes". Retrieved 11 January 2018.
  14. Russ Cox. "Go 1.3+ Compiler Overhaul". Retrieved 11 January 2018.
  15. "Js_of_ocaml". Retrieved 8 October 2014.
  16. "J2Eif Research Page - Chair of Software Engineering". doi:10.1007/978-3-642-21952-8_4. Retrieved 2014-07-08.
  17. "C2Eif Research Page - Chair of Software Engineering". Retrieved 2014-07-08.
  18. "Swiftify Objective-C to Swift Converter". Retrieved 2017-11-14.
  19. "Runtime Converter". Retrieved 2017-11-14.
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