elm-format-wrapper that allows to ignore blocks of code

elm-format-wrapper.nix

{ pkgs }:

pkgs.symlinkJoin {
  name = "elm-format-wrapper";
  paths = [
    (pkgs.writers.writeDashBin "elm-format" ''
      # elm-format-wrapper 1.0.0
      #
      # Usage:
      #   elm-format [--help] [--action=ACTION] PATH...
      #
      # Description:
      #   Format paths like elm-format, but allow ignoring blocks of code:
      #
      #     --elm-format-ignore-begin
      #     don't format code here...
      #     --elm-format-ignore-end
      #
      # Available options:
      #   --action=format (this is the default action)
      #       reformat each file
      #
      #   --action=diff
      #       show patch that would be applied if action would be "format"
      #       instead of formating the files.
      #
      #   -h, --help
      #       show this help :)
      #
      #   --output=PATH
      #   --output PATH
      #       write formatted file to PATH.
      #       If multiple files are formatted, then the last one will end up in
      #       the output path.  This option is mainly for compatibility with
      #       elm-format.
      #
      #   --yes
      #       ignored; this option is for compatibility with elm-format
      #     

      set -efu

      help() {
        ${pkgs.gnused}/bin/sed -rn '
          0,/^$/{
            /#!/d
            s/^# ?//p
          }
        ' "$0"
      }

      action=format
      output=
      for arg; do
        shift
        case $arg in
          --action=*)
            action=''${arg#*=}
            case $action in
              diff) continue;;
              format) continue;;
            esac
            echo "error: bad action: $action" >&2
            exit 1
            ;;
          --output)
            output=$1
            shift
            continue
            ;;
          --output=*)
            output=''${arg#*=}
            continue
            ;;
          --yes)
            continue
            ;;
          -h|--help)
            help
            exit
            ;;
          -*)
            echo "error: bad argument: $arg" >&2
            help >&2
            exit 1
        esac
        set -- "$@" "$arg"
      done

      action_format() {(
        old=$1
        new=$2
        if test -n "$output"; then
          ${pkgs.coreutils}/bin/cp "$new" "$output"
        else
          ${pkgs.coreutils}/bin/cp "$new" "$old"
        fi
      )}

      action_diff() {(
        old=$1
        new=$2
        ${pkgs.diffutils}/bin/diff -u --color "$old" "$new"
        if test -n "$output"; then
          ${pkgs.coreutils}/bin/cp "$new" "$output"
        fi
      )}

      temp=$(${pkgs.coreutils}/bin/mktemp -t elm-format-wrapper.XXXXXXXX)
      clean() {
        ${pkgs.coreutils}/bin/rm "$temp"
      }
      trap clean EXIT

      ${pkgs.coreutils}/bin/yes n |
      ${pkgs.elmPackages.elm-format}/bin/elm-format "$@" |
      ${pkgs.gnused}/bin/sed -rn '
        /^This will overwrite the following files to use Elm.s preferred style:$/,/^This cannot be undone!/{
          s/^\s*(\S+)$/\1/p
        }
      ' |
      while IFS= read -r path; do
        ${pkgs.coreutils}/bin/cat "$path" |
        ${pkgs.gnused}/bin/sed -r '
          /^--elm-format-ignore-begin$/,/^--elm-format-ignore-end$/{
            s/^/--@/
          }
        ' |
        ${pkgs.elmPackages.elm-format}/bin/elm-format --stdin > "$temp"
        ${pkgs.gnused}/bin/sed -ir '
          /^--@--elm-format-ignore-begin$/,/^--@--elm-format-ignore-end$/{
            s/^--@//
          }
        ' "$temp"

        action_"$action" "$path" "$temp"
      done
    '')
  ];
}

The above Nix package can be used to create a shell providing a wrapped elm-format, e.g. like so:

$ nix-shell -p '
  (import <nixpkgs> {}).callPackage (builtins.fetchurl {
    url = https://gist.githubusercontent.com/4z3/e477cfbedb05b560940d26fbd80b01ab/raw/53e79fdabcd956cb797b7671959cf65b6e6d44c1/elm-format-wrapper.nix;
 }) {}
'

(Only tested with nixpkgs 20.09, so YMMV)

wrapper by itself:

#!/usr/bin/env bash

# elm-format-wrapper 1.0.0
#
# Usage:
#   elm-format-wrapper [--help] [--action=ACTION] PATH...
#
# Description:
#   Format paths like elm-format, but allow ignoring blocks of code:
#
#     --elm-format-ignore-begin
#     don't format code here...
#     --elm-format-ignore-end
#
# Available options:
#   --action=format (this is the default action)
#       reformat each file
#
#   --action=diff
#       show patch that would be applied if action would be "format"
#       instead of formating the files.
#
#   -h, --help
#       show this help :)
#
#   --output=PATH
#   --output PATH
#       write formatted file to PATH.
#       If multiple files are formatted, then the last one will end up in
#       the output path.  This option is mainly for compatibility with
#       elm-format.
#
#   --yes
#       ignored; this option is for compatibility with elm-format
#     
set -efu
help() {
  gsed -rn '
    0,/^$/{
      /#!/d
      s/^# ?//p
    }
  ' "$0"
}
action=format
output=
for arg; do
  shift
  case $arg in
    --action=*)
      action=''${arg#*=}
      case $action in
        diff) continue;;
        format) continue;;
      esac
      echo "error: bad action: $action" >&2
      exit 1
      ;;
    --output)
      output=$1
      shift
      continue
      ;;
    --output=*)
      output=''${arg#*=}
      continue
      ;;
    --yes)
      continue
      ;;
    -h|--help)
      help
      exit
      ;;
    -*)
      echo "error: bad argument: $arg" >&2
      help >&2
      exit 1
  esac
  set -- "$@" "$arg"
done
action_format() {(
  old=$1
  new=$2
  if test -n "$output"; then
    cp "$new" "$output"
  else
    cp "$new" "$old"
  fi
)}
action_diff() {(
  old=$1
  new=$2
  diff -u --color "$old" "$new"
  if test -n "$output"; then
    cp "$new" "$output"
  fi
)}
temp=$(mktemp -t elm-format-wrapper.XXXXXXXX)
clean() {
  rm "$temp"
}
trap clean EXIT
yes n |
elm-format "$@" |
gsed -rn '
  /^This will overwrite the following files to use Elm.s preferred style:$/,/^This cannot be undone!/{
    s/^\s*(\S+)$/\1/p
  }
' |
while IFS= read -r path; do
  cat "$path" |
  gsed -r '
    /^--elm-format-ignore-begin$/,/^--elm-format-ignore-end$/{
      s/^/--@/
    }
  ' |
  elm-format --stdin > "$temp"
  gsed -ir '
    /^--@--elm-format-ignore-begin$/,/^--@--elm-format-ignore-end$/{
      s/^--@//
    }
  ' "$temp"
  action_"$action" "$path" "$temp"
done