VSCode for Haskell Development

Haskell is a purely functional programming language. The VSCode extension Haskell for Visual Studio Code has been configured to develop Haskell code in the Docker container remotely. This blog explains how to use the VSCode editor to develop Haskell programs. The features, where you can find the table, summarises the features that HLS supports. Many of these are standard LSP features.

Install using docker

Create the following Dockerfile1:


FROM ubuntu:20.04
ARG USERNAME=ojitha
ARG USER_UID=1000
ARG USER_GID=$USER_UID

# Create the user
RUN groupadd --gid $USER_GID $USERNAME \
    && useradd --uid $USER_UID --gid $USER_GID -m $USERNAME \
    && apt-get update \
    && apt-get install -y sudo \
    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME \
    && chmod 0440 /etc/sudoers.d/$USERNAME \
    && apt-get install build-essential curl libffi-dev libffi7 libgmp-dev libgmp10 libncurses-dev libncurses5 libtinfo5 -y
    
# This is necessary to attach VSCode later
USER $USERNAME
WORKDIR /home/$USERNAME

To create image

docker build -t ojhaskell .

After the image created, create the docker instance as follows:

docker run -it ojhaskell

You will get the bash shell to install the GhCup2. Execute the following command.

curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | sh

In the installation process, you will get the following questions to answer; you can accept the default by pressing the enter key. However, HLS default is N, which should be Y because we need to HLS to run the Haskell extension in the VSCode.

Install GhCup
Install GhCup

Installation may take time. At the end you may get the following message if everything is successful:

Haskell installation message
Haskell installation message

If the message is different, you have to add the contents of the /home/$USERNAME/.ghcup/env to the .bashrc file. When you type ghci, it should give you a Haskell prompt.

For example:

# add by ojitha
case ":$PATH:" in
    *:"/home/ojitha/.ghcup/bin":*)
        ;;
    *)
        export PATH="/home/ojitha/.ghcup/bin:$PATH"
        ;;
esac
case ":$PATH:" in
    *:"$HOME/.cabal/bin":*)
        ;;
    *)
        export PATH="$HOME/.cabal/bin:$PATH"
        ;;
esac

To verify all the components are install run

ghcup tui

which will give you

Installed GhCup components
Installed GhCup components

At least above components should be installed.

VSCode development

Install the VSCode docker extension. In the docker

Attach VSCode
Attach VSCode

As shown above, you can right-click ojhaskell and attach VSCode. That will open a new instance of the VSCode.

Most of cases, Ubuntu use dash shell instead of bash shell.

In Ubuntu, bin/sh is a dash shell. Run the following command to verify your shell

ls -l `which sh`
# lrwxrwxrwx 1 root root 4 Jul 18  2019 /usr/bin/sh -> dash

If dash, run the sudo dpkg-reconfigure dash to change this to bash shell as follows.

Change shells
Change shells

Close the current VSCode and reattach VSCode again. You are ready to install the VSCode Haskell extension3.

Complete Docker image for Haskell

All the above manual operation can be included in the Dockerfile as follows:

FROM ubuntu:20.04
ARG USERNAME=ojitha
ARG USER_UID=1000
ARG USER_GID=$USER_UID

# Create the user
RUN groupadd --gid $USER_GID $USERNAME \
    && useradd --uid $USER_UID --gid $USER_GID -m $USERNAME \
    && apt-get update \
    && apt-get install -y sudo \
    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME \
    && chmod 0440 /etc/sudoers.d/$USERNAME 

RUN apt-get install build-essential curl vim libffi-dev libffi7 libgmp-dev libgmp10 libncurses-dev libncurses5 libtinfo5 -y

USER $USERNAME
WORKDIR /home/$USERNAME


# # ==================================
#   *  any nonzero value for noninteractive installation
ENV BOOTSTRAP_HASKELL_NONINTERACTIVE=1
#   *  any nonzero value to not trigger the upgrade
ENV BOOTSTRAP_HASKELL_NO_UPGRADE=0
#   * any nonzero value for more verbose installation
ENV BOOTSTRAP_HASKELL_VERBOSE=0
#   *  the ghc version to install
ENV BOOTSTRAP_HASKELL_GHC_VERSION=9.2.7
#   *  the cabal version to install
ENV BOOTSTRAP_HASKELL_CABAL_VERSION=3.6.2.0
#   *  whether to install latest hls
ENV BOOTSTRAP_HASKELL_INSTALL_HLS=Y
# # ==================================

#   *  any nonzero value to only install ghcup
# ENV BOOTSTRAP_HASKELL_MINIMAL=0
#   *  any nonzero value to respect The XDG Base Directory Specification
# ENV GHCUP_USE_XDG_DIRS=1
#   * BOOTSTRAP_HASKELL_INSTALL_NO_STACK - disable installation of stack
#   * BOOTSTRAP_HASKELL_INSTALL_NO_STACK_HOOK - disable installation stack ghcup hook
#   * BOOTSTRAP_HASKELL_ADJUST_BASHRC - whether to adjust PATH in bashrc (prepend)
#   * BOOTSTRAP_HASKELL_ADJUST_CABAL_CONFIG - whether to adjust mingw paths in cabal.config on windows
#   * BOOTSTRAP_HASKELL_DOWNLOADER - which downloader to use (default: curl)
#   * GHCUP_BASE_URL - the base url for ghcup binary download (use this to overwrite https://downloads.haskell.org/~ghcup with a mirror)

RUN ["/bin/bash", "-c", "curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | sh"]
RUN ["/bin/bash", "-c", "echo '# add by $USER' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo 'case \":$PATH:\" in' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '    *:\"$HOME/.ghcup/bin\":*)' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        ;;' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '    *)' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        export PATH=\"$HOME/.ghcup/bin:$PATH\"' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        ;;' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo 'esac' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo 'case \":$PATH:\" in' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '    *:\"$HOME/.cabal/bin\":*)' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        ;;' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '    *)' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        export PATH=\"$HOME/.cabal/bin:$PATH\"' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '        ;;' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo 'esac ' >> ~/.bashrc"] 
RUN ["/bin/bash", "-c", "echo '' >> ~/.bashrc"] 

# set bash to default
RUN echo "dash dash/sh boolean false" | sudo debconf-set-selections
RUN sudo dpkg-reconfigure -f noninteractive dash

This will simplify your process of creating the Haskell development environment. I set the minimum environment variables4 create the same above image.

To create the image from the above Dockerfile:

docker build --no-cache -t testhaskell .

When after image has been created run the following command to create a container:

docker run -it --rm -v "./work":/home/ojitha/work testhaskell

You can attach VSCode to this instance without any further configurations.

Features

Some of the features are:

  • Shows ingoing and outgoing calls for a function.
    Call hierarchy
    Call hierarchy
  • Apply Hlint fixes
    Apply Hlint fixes
    Apply Hlint fixes
  • Qualify imported names
    Qualify imported names
    Qualify imported names
  • Fold definition
    Fold definition
    Fold definition

References:

  1. Add a non-root user to a container ↩

  2. CHCup ↩

  3. Haskell for Visual Studio Code ↩

  4. bootstrap-haskell ↩

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