froid

June 4, 2026 · View on GitHub

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:

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.
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.

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.

froid

A counter, in full

Building

1. Build the Frege compiler (one-time)

2. Build and run a sample

Status

License