The compiler and all cross-compilers come from the haskell.nix input pinned in flake.lock. The ghc914-sh compiler is the stable-haskell fork, not the stock GHC 9.14 release from GHCup. Use the same source checkout and lock file for every platform; Android ARMv7 does not require a separate branch or tag.

compiler-selection.nix matches Hydra's compiler build context so the pinned compiler can use its existing cache. That context is scoped to the compiler and its build tools; application dependencies keep the project's current packages.

Run these commands from the repository root:

make help
make nix-list                  # list this host's existing Hydra outputs
make nix-native                # native library and CLI
make nix-android               # ARM64 + ARMv7 libraries, on x86_64-linux
make nix-windows               # Windows library and CLI, on x86_64-linux
make nix-linux-static          # x86_64, i686, and ARM64 static Linux CLIs
make nix-build                 # all existing outputs for this build host

Each command builds this checkout, without cloning another branch. Output links are placed under result-nix/<build-system>/<package>. ZIP filenames inside the outputs retain the existing Hydra names. For example, the ARMv7 client library is result-nix/x86_64-linux/armv7a-android:lib:simplex-chat/pkg-armv7a-android-libsimplex.zip. Nix includes tracked working-tree changes; add new source files to Git before building so they are included.

For a remote builder, select its build system explicitly. Nix must already have that builder configured:

make nix-ios NIX_SYSTEM=aarch64-darwin
make nix-darwin-libs NIX_SYSTEM=x86_64-darwin
make nix-build NIX_SYSTEM=aarch64-linux

nix-ios produces the Haskell static-library ZIP. It does not run Xcode, sign an iOS application, or upload to TestFlight. ARM64 device libraries and Intel simulator libraries retain their separate outputs. The existing iOS preparation scripts retag ARM64 archives with mac2ios -s when an Apple Silicon simulator is needed.

The Cabal v2 builder stores ordinary package archives in its staged store. link-mobile-library.sh relinks the selected package and its dependencies before that store is pruned. iOS ZIPs contain both the plain libHSsimplex-chat-*.a and the rolled libHSsimplex-chat-*-ghc*.a used by update-pbxproj.sh. Android and Windows clients link the threaded RTS into libsimplex.so or libsimplex.dll; Android's separate libsupport.so resolves RTS symbols from the client library. ARM64 Android libraries retain the 16 KiB linker alignment. These steps run in the derivation's build/install phases because v2 component overrides do not run the older postBuild and postInstall hooks.

NIX_ARGS passes additional options to nix build, and NIX_OUT_DIR changes the output-link directory. Each derivation has a two-hour timeout by default; set NIX_BUILD_TIMEOUT in seconds to change it. For a wall-clock limit on a local batch, use timeout from GNU coreutils to include environment setup in the limit:

timeout 6h nix develop -c make nix-build
make nix-native NIX_ARGS=--dry-run

.github/workflows/nix.yml runs these same outputs on Linux ARM64/x86_64 and Darwin ARM64/x86_64, with a six-hour job limit. It uploads the existing library ZIPs and CLIs under an artifact named for the source SHA. Hydra continues to consume the same hydraJobs attributes. The older application release workflow still uses its existing GHCup/Docker compiler settings; migrating that packaging and release path is separate from this Nix build lane.

To build both Android APK ABIs from the current checkout, preserving the same compiler and source, use the existing builder's source/skip-checkout flags:

scripts/android/build-android.sh -sg VERSION_CODE

This command also needs the Android SDK, Gradle, and its other existing tools. The APK reproduction script now checks out the supplied tag for both ABIs. Historical releases that used a separate -armv7a tag must be reproduced with their historical build scripts.

Updating package hashes

Install nix, gawk and jq.
Start nix-shell from repo root:

nix-shell -p nix-prefetch-git

Run in nix shell:

gawk -f ./scripts/nix/update-sha256.awk cabal.project > ./scripts/nix/sha256map.nix