How to build other packages for wasm32?

[brakubraku]

Trying to add some dependencies to the project (entropy package specifically), but I am running into problems, which I assume stem from building for wasm32 architecture.

Failed to build entropy-0.4.1.10-7ad7e63631182fc592b386e6fc995366bf33c32811197665f528cdb53790f8d9. The failure occurred during the configure step. The exception was:
/home/branko/.ghc-wasm/.cabal/logs/ghc-9.10.0.20240509/entropy-0.4.1.10-7ad7e63631182fc592b386e6fc995366bf33c32811197665f528cdb53790f8d9.log: withFile: user error (CallStack (from HasCallStack):
withMetadata, called at src/Distribution/Simple/Utils.hs:447:12 in Cabal-3.11.0.0-inplace:Distribution.Simple.Utils
Error: cabal:
'/nix/store/p64q67ynh6aiblhlca4gdc2ycc6q6p8g-wasm32-wasi-ghc-9.10/bin/wasm32-wasi-ghc'
exited with an error:
wasm-ld: error: unable to find library -lHSrts-1.0.2_thr
wasm32-wasi-clang: error: linker command failed with exit code 1 (use -v to
see invocation)
wasm32-wasi-ghc-9.10.0.20240509: wasm32-wasi-clang' failed in phase Linker'.
(Exit code: 1)

I wonder if there's any generic "step-by-step" of how to update a package for it to compile to wasm32 without issues?

[brakubraku]

I opened another issue in jsaddle-wasm, which I believe will help me resolve this.

[amesgen]

Hey, thanks for opening the issue!

In your concrete case, the problem is that entropy has uses Cabal's build-type: Custom and therefore a Setup.hs script that has to be executed before building the package. This is not supported by the WASM backend as it is a cross compiler, also see haskell/entropy#42.

One solution for this specific case is to simply change entropy to use build-type: Simple instead: haskell/entropy@main...amesgen:entropy:build-type-simple
You can use this e.g. like this in your cabal.project:

source-repository-package
  type: git
  location: https://github.com/amesgen/entropy
  tag: 98dd0cf23355a1f87af14c10098b9545605ac556

Hope that helps!

---

I wonder if there's any generic "step-by-step" of how to update a package for it to compile to wasm32 without issues?

Yeah, that sounds nice; here a (probably non-exhaustive) list of some of the things that can cause a package to not compile via the WASM backend:

• The packages uses build-type: Custom
  • entropy, as discussed above
  • Another common example is pretty-simple: Cross-compilation broken by custom Setup.hs used for doctests cdepillabout/pretty-simple#82
• Issues with FFI. Often, C FFI (e.g. bindings to C libraries or optimized implementations) work out of the box, but sometimes, one has to make minor changes (that often aren't really WASM-specific):
  • Fixing FFI signatures:
    • rts_getThreadId: fix return type on newer GHCs haskell-unordered-containers/hashable#263
    • Correct signatures for FinalizerPtrs haskell/zlib#57
    • FFI changes to support the GHC WASM/WASI backend corsis/clock#85
    • Correct signature for fclose snoyberg/yaml#214
  • FFI uses features not available on WASM:
    • Add WASI initialization support haskellari/splitmix#73
• The library doesn't support 32bit architectures (like wasm32):
  • Fix 32 bit compilation well-typed/cborg#322
• There already is compatibility code for non-standard platforms, it just needs to be enabled:
  • Use polling as a generic fallback haskell-fswatch/hfsnotify#110
• Class instances/functionality for platform-specifc types needs to be guarded behind CPP:
  • Support targets without signals (like WASI) ekmett/lens#1020
  • Make Posix instances conditional for non-standard platforms chessai/semirings#86
  • Support targets without signals (like WASI) dreixel/generic-deriving#91
• Code fails at runtime because unsupported features are used (often happens because e.g. the WASM-build unix library chooses to turn compile-time into runtime errors, which is convenient, but can also be surprising):
  • Don't crash when home directories are unsupported hspec/hspec#834
  • Only install handlers when HAVE_SIGNAL_H UnkindPartition/tasty#365
• No Template Haskell support: https://gitlab.haskell.org/ghc/ghc/-/issues/24376
  • Miso: dmjio/miso@master...amesgen:miso:remove-th

---

Also, in case you are interested, feel free to join the GHC WASM Matrix channel: https://matrix.to/#/#haskell-wasm:matrix.terrorjack.com