Firstly a big thanks to Neil Mix for this threading library... http://www.neilmix.com/2007/02/07/threading-in-javascript-17/ It is just what GHCJS needs for threading support (at least in Firefox). Here is what I did to get it working in GHCJS... * Added "Jump" so that tail calls don't grow the stack * Implemented a really basic scheduler * Added some thread and MVar primitives * Added a YieldTrampoline calling convention to GHCJS The code is here https://github.com/hamishmack/ghcjs cabal install cd examples runhaskell BuildTestYield.hs ../../ghc-7.0.2 Then open test-yield.html in Firefox. I think you can build it with any GHC 7 (I used 7.0.3), but you need the 7.0.2 source. I get this error when I try to use the 7.0.3 source... ../../ghc-7.0.3/libraries/base/GHC/Unicode.hs:35:0: error: HsBaseConfig.h: No such file or directory