Imagine being able to add a field to the config form with just one line! It can be done if you use code generation.

Use ConfigFormGenerator in your ConfigSchema.elm to make a Config.elm file (it can be excluded from your src/ directory if you want, since it won't be compiled directly with your other elm files):

-- ConfigSchema.elm




import ConfigFormGenerator exposing (Kind(..))
import Html exposing (Html)


myConfigFields : List ( String, Kind )
myConfigFields =
    [ ( "Header Font Size", IntKind "headerFontSize" )
    , ( "Body Font Size", IntKind "bodyFontSize" )
    , ( "Background Color", ColorKind "bgColor" )
    ]


main : Html msg
main =
    let
        generatedElmCode =
            ConfigFormGenerator.toFile myConfigFields


        _ =
            Debug.log generatedElmCode ""
    in
    Html.text ""

When compiled, it makes an elm app whose sole purpose is to console.log the elm code needed for a Config.elm file. To generate it, run something like this:

# Compile schema file to tmp js:
elm make ConfigSchema.elm --output=~/tmp/tmp.js > /dev/null

# Run compiled js with node, which logs out generated elm code, and save to Config.elm:
node ~/tmp/tmp.js > Config.elm 2>/dev/null

How to automate with a watcher script

#!/bin/bash

CONFIG_SCHEMA_ELMFILE=ConfigSchema.elm
CONFIG_ELMFILE=Config.elm
TMP_JS=~/tmp/gen-config.js
MAIN_ELMFILE=Main.elm
SERVER_DIR=public/
MAIN_JS_OUTPUT=public/js/main.js

GENERATE_ARGS="$CONFIG_SCHEMA_ELMFILE $TMP_JS $CONFIG_ELMFILE"

# Command for generating Config.elm from ConfigSchema.elm
generate_config () {
  CONFIG_SCHEMA_ELMFILE=$1
  TMP_JS=$2
  CONFIG_ELMFILE=$3

  # Use `elm make` to make an elm app that console.logs the generated Config.elm code
  elm make $CONFIG_SCHEMA_ELMFILE --output=$TMP_JS > /dev/null && \
    # Run it with `node` to print the output and write to Config.elm
    node $TMP_JS > $CONFIG_ELMFILE 2>/dev/null
}
export -f generate_config

# Generate the config initially, just in case it doesn't exist
generate_config $GENERATE_ARGS

# Watch for config changes
chokidar $CONFIG_SCHEMA_ELMFILE --command "generate_config $GENERATE_ARGS" &

# Watch for elm changes
elm-live $MAIN_ELMFILE --dir=$SERVER_DIR -- --output=$MAIN_JS_OUTPUT &

wait

This will watch for changes to ConfigSchema.elm and generate a Config.elm file. Make sure you have the following installed, too:

# (use --save-dev instead of --global if you only need it locally for one project)
npm install --global elm elm-live@next chokidir