はじめまして！ (^-^)

• 山本悠滋 (@igrep)
• Haskell歴 ≒ プリキュアおじさん歴 ≒ 約7年。
• 趣味Haskeller兼仕事Haskeller @ IIJ-II 😄
• igrep.elというEmacsプラグインがありますが無関係です！

宣伝 hask(_ _)eller

• 2017年から日本Haskellユーザーグループというコミュニティーグループを立ち上げ、その発起人の一人として活動しております。
  • 愛称: Haskell-jp
• SlackのワークスペースやSubreddit、ブログ、Wikiなど、いろいろ運営しております。

Topics and Summary

※From this slide, written in English to show the developer of Asterius.

• 🔍What Asterius is.
• ⚙️How Asterius works.
• 💪I tried to run a function I wrote in Haskell before on a browser.
  • 😥But in vail due to a bug of Asterius’s FFI.

🔍What Asterius is:

• A Haskell to WebAssembly compiler.
• Still in heavy development.
• All GHC features except Template Haskell and most standard IO functions should work.
• Provides FFI to call JavaScript from Haskell, and Haskell from JavaScript, vice versa.
  • Call JavaScript functions to perform some IOs.
• Now provides ahc-cabal command to build a module dependent on external packages.

👍Asterius’s Pros

• Output wasm is relatively small (considering it contains a runtime of Haskell).
  • Empty project:
    • 36KB (only .wasm file).
    • 168KB (including uncompressed .mjs files).
  • 😕My app:
    • 1.9MB (only .wasm file).
    • 2.1MB (including uncompressed .mjs files).

👍Asterius’s Pros (cont)

• Available with HEAD of GHC!
  • Forked from GHC. But the diff from original GHC is so small that the developer can rebase regularly.

👎Asterius’s Cons

• Still very unstable and buggy.
• Most standard IO functions and Template Haskell aren’t available.
  • Call JavaScript functions to perform some IOs.
  • Currently few wrappers around JavaScript functions are provided unless declaring foreign import javascript explicitly…

👎Asterius’s Cons (cont)

• Only works on V8 and SpiderMonkey
  • The runtime JavaScript modules depend on BigInt.
  • Use Firefox Nightly or Chrome!

⚙️How Asterius works

Ref: IR types and transformation passes

1. Wrap the fork of GHC with a frontend plugin:
   • Convert .cmm files to AsteriusModule (custom object format for Asterius).
2. ahc-ld (dedicated linker) links for WASM.

⚙️How Asterius works (cont)

3. ahc-dist makes it executable.
   • With binaryen or wasm-toolkit, validate and convert the modules to WASM module.
   • Copy JavaScript modules required at runtime.
   • Emit the entry JavaScript module to run the main function of the source Haskell file.

How I tried to run my function with Asterius

The target app:

• Parse a simple line-separated file to sum up my expenditure.
• Tried to export the core function typed as
  FilePath -> Text -> Text using FFI.

How I tried to run my function with Asterius (cont)

🔥I had to drop some dependent packages to compile my code…

• To avoid to depend on Template Haskell and IO.
• 😨If any of the dependency depends on them, fails to build.

How I tried to run my function with Asterius (cont)

• But avoiding IO is actually easy by believing in types!
• Haskell is very good at writing pure functions!
• In Haskell, IO is always explicitly typed as IO:
  • Easily detect which functions do IO, then split them out!
  • ✨Impure functions don’t spark joy!

Result

• Successfully built a module exporting the core function.
• But finally got stuck by a bug of FFI.
  • The foreign export javascripted function doesn’t return the expected result!
• Perhaps I can avoid by not using foreign export javascript, but left so far.

✅Summary

• Asterius is a Haskell to WebAssembly compiler.
• Now we can run various Haskell code on a browser!
  • But currently we have to drop much code to compile…