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daml/ghc-lib/new-working-on-ghc-lib.md
Claudio Bley 02e8826b6e
Make building ghc in-project and with Bazel work again (#13970) 
* Pass `with_bazel` flag to cabal when building Hadrian inside ghc

* Update GHC

with digital-asset/ghc#121

* Bring back documentation how to test syntax changes

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2022-05-30 09:47:44 +02:00

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Copyright (c) 2022 Digital Asset (Switzerland) GmbH and/or its affiliates. All Rights Reserved. SPDX-License-Identifier: (Apache-2.0 OR BSD-3-Clause)

Working on ghc-lib

If you need to build, test, deploy or develop ghc-lib as used by Daml and utilizing the Digital Asset GHC fork these notes are for you.

Here are instructions for when working on Daml surface syntax, as implemented in the the digital-assert fork of ghc. (linked in via ghc-lib, see ghc-lib.

Cloning the digital-assert fork of ghc

  1. Make initial clone from the main ghc gitlab repo:
git clone --recurse-submodules https://gitlab.haskell.org/ghc/ghc.git
cd ghc
  1. Add the DA fork as a remote
git remote add da-fork git@github.com:digital-asset/ghc.git
git fetch da-fork
  1. Checkout the version of interest and update the submodules:
git checkout da-master-8.8.1
git submodule update --init --recursive

Iterating on parser/desugaring in ghc

Working locally in a branch from da-master-8.8.1, there are two files which generally need changing to update syntax and desugaring:

The quickest way to build and test is:

  1. hadrian/build.sh --configure --flavour=quickest -j

  2. ./_build/stage1/bin/ghc ./Example.hs -ddump-parsed | tee desugar.out

Step 1 gives immediate feedback on build failures, but takes about 2-3 minutes when successful. For Step 2 you need a Daml example file. The input file must end in .hs suffix. It must begin with the pragma: {-# LANGUAGE DamlSyntax #-}

Interactive development workflow

While working on GHC, you can integrate your changes directly into the daml project as follows.

  1. Make initial clone from the main ghc gitlab repo:

    git clone --recurse-submodules https://gitlab.haskell.org/ghc/ghc.git
cd ghc

  2. Add the DA fork as a remote

    git remote add da-fork git@github.com:digital-asset/ghc.git
git fetch da-fork

  3. Checkout the version of interest (usually GHC_REV from [/bazel_tools/ghc-lib/version.bzl]) and update the submodules:

    git checkout da-master-8.8.1
git submodule update --init --recursive

  4. Add WORKSPACE and BUILD file:

    cp ../daml/bazel_tools/ghc-lib/BUILD.ghc BUILD
touch WORKSPACE

  5. Make changes... 

  6. Build referencing your local checkout of GHC:

    cd ../daml
bazel build --override_repository=da-ghc="$( cd ../ghc ; pwd )" //...

After you are satisfied with your changes, just open a PR on the GHC repository and after it is merged update the SHA value in GHC_REV in [/bazel_tools/ghc-lib/version.bzl] and create a PR for the daml project.