uncaught exception in Haskell main thread: Error: Cannot create a string longer than 0x1fffffe8 characters

[o1lo01ol1o]

During template haskell linking on our frontend (obelisk), we apparently hit thm hard limit in node.js for the maximum length of a string.

I opened an issue in ghcjs (ghcjs/ghcjs#842 ), but given the js backend in 9.12 that reflex is moving to, I'm not sure anyone would even look at a new issue raised in the GHCJS repo before that upgrade lands here. So, two questions:

Is there a workaround in GHCJS 8.10.7?
Will the new js backend also have this limit?

[ymeister]

@o1lo01ol1o Good questions! I kind of doubt any effort would be made towards fixing problems on 8.6/8.10 at this point. Do you have a small & isolated example that I could test on 9.12?

[o1lo01ol1o]

I don't really have a small example. According to GHC's dump-splices, we're generating 53,000 lines of Haskell code from our template haskell. You could probably drum up a stress test by code-generating a huge, tree like GADT and derivingJSONGADT, GEq, Gord, GSHow, and ArgDict on it.

EDIT This only counts TH in our local modules. I assume TH generated by any imported deps also count.

However, I'm not sure how the linker handles TH modules, so it may also be affected by the total symbol count exported and the total number of modules and their import sites.

[maralorn]

@o1lo01ol1o This was the reason why we switched to the new js/wasm backend. We didn’t encounter the issues there.

[o1lo01ol1o]

@maralorn @ymeister I'm opening this again because the problem is not restricted to Template Haskell: it is a consequence of merely linking our large common and frontend. libHSfrontend-0.1-inplace-ghc8.10.7.a on this project is ~680mb.

So, more questions:

1. How tightly coupled is the node runtime used by ghcjs? pinning an older or newer runtime might buy us another few hundred MBs.
2. How far out is ios support for the new 9.12 js/wasm backend?

[ymeister]

@maralorn @ymeister I'm opening this again because the problem is not restricted to Template Haskell: it is a consequence of merely linking our large common and frontend. libHSfrontend-0.1-inplace-ghc8.10.7.a on this project is ~680mb.

So, more questions:

1. How tightly coupled is the node runtime used by ghcjs?  pinning an older or newer runtime might buy us another few hundred MBs.

2. How far out is ios support for the new 9.12 js/wasm backend?

1. Sorry, I can't answer this question with any degree of confidence. In worst of the worst case, you may want to try to use overlays for that.
2. As far as possible. The general plan of it is that js/wasm are released first even without mobile support, then android likely follows with ios likely to be last. Although, I am aware of several people that IIRC are working towards ios builds with new backend, so it might be sooner than expected.