froid

Write Android apps in the Frege programming language (Haskell on the JVM) — pure Frege, no Kotlin, with much less ceremony than the original thanks to Frege's new automated FFI.

froid is two things:

1. A general Frege-on-Android library — thin handles over android.view / android.widget / android.app, with the boilerplate erased by auto-FFI and the irreducible glue (the Activity, callbacks) hidden in the library. App code writes no Java and no Kotlin.
2. A declarative UI in the Elm Architecture, backed by FRP — built on the View system. You write update :: Msg -> Model -> Model and view :: Signal Model -> Ui Msg; froid keeps the views in sync.

A counter, in full

module app.Counter where

import froid.app.Activity
import froid.ui.App
import froid.frp.Signal

native module type Activity where {}      -- the one line that makes this an Activity

data Msg = Increment

update :: Msg -> Int -> Int
update Increment n = n + 1

view :: Signal Int -> Ui Msg
view model = column
    [ dynText (fmap (\n -> "Count: " ++ show n) model)
    , button "Increment" Increment
    ]

counter :: App Int Msg
counter = App { initial = 0, update = update, view = view }

onCreate :: Activity -> IO ()
onCreate this = runApp this counter

Building

1. Build the Frege compiler (one-time)

froid needs the auto-FFI compiler from Frege master (PR #400; no release has it yet). The script pins a known-good commit and vendors the jar to libs/.

Requirements: a JDK in the 8–17 range (17 recommended), make, curl, jar. byacc is not needed.

JAVA_HOME=/path/to/jdk17 tools/build-frege-snapshot.sh

2. Build and run a sample

./gradlew :geoquiz:assembleDebug
./gradlew :geoquiz:installDebug      # onto a running emulator/device
# or open in Android Studio and press Run

compileFrege<Variant> (from froid-gradle-plugin) runs the Frege compiler over each module's src/frege, generating Java that AGP compiles and dexes.

Status

Builds and runs. :geoquiz and :counter assemble to APKs and run on an emulator (AGP 8.13, compileSdk 36): the question renders, True/False checks the answer with color-coded feedback, and Prev/Next navigate — all driven by the Frege FRP.

Roadmap: more widgets (EditText, images, lists with stable keys); the deeper FRP correctness items (subscription disposal for rotation, transactional glitch-freedom) are noted but not needed by the current static view tree; folding more of the original View bindings back in where useful (they remain in git history on master).

Frege master is pinned at commit 067cad057385d36134812bde3fc1b453e5274855.

License

Apache License 2.0. froid is © Michael Chavinda and contributors.