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Update docs for elm-pages scripts.
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Dillon Kearns
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# The `elm-pages run` CLI command
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`elm-pages` provides this command to help you scaffold new code. The starter template includes a codegen module for scaffolding new Route Modules. However, you can use it to build your own custom generators to scaffold any kind of Elm code you can think of! Indeed, you're not even limited to generating Elm code (or generating code at all). The `elm-pages run` command really just takes an Elm module [Elm `worker` application](https://package.elm-lang.org/packages/elm/core/latest/Platform#worker) and runs it.
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## Executing an `elm-pages` Script
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If you have a file called `codegen/Hello.elm`
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```elm
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module Hello exposing (main)
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main : Program () () msg
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main =
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Platform.worker
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{ subscriptions = \_ -> Sub.none
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, update = \_ _ -> ( (), Cmd.none )
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, init =
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\_ ->
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let
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_ =
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Debug.log "Greeting" "Hello from Elm!
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in
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( (), Cmd.none )
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}
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```
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`elm-pages` will compile and run this Elm `worker` app when we execute this from the command line:
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```
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npx elm-pages codegen Hello
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Greeting: "Hello from Elm!"
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```
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The command uses [`elm-codegen`](https://github.com/mdgriffith/elm-codegen), which is both a command line tool and an Elm package to help you generate Elm code.
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##
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examples/docs/content/docs/11-elm-pages-scripts.md
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examples/docs/content/docs/11-elm-pages-scripts.md
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---
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description: You can run scripts written in pure Elm with the `elm-pages run` CLI command.
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---
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# `elm-pages` Scripts
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The `elm-pages run` command lets you use [`BackendTask`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/BackendTask)'s to do scripting tasks with Elm. The goal is to make it as simple as possible to write a Script in Elm and run it from the command line. You can use any of the techniques from BackendTask's to read files, make HTTP requests, get environment variables, or async NodeJS functions through [`BackendTask.Custom.run`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/BackendTask-Custom#run).
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## Quick Start
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The [elm-pages starter repo](https://github.com/dillonkearns/elm-pages-3-alpha-starter) comes with a `script/` folder that is setup with an `elm.json` with `elm-pages` as a dependency, but you can also create the `script/` folder from scratch:
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```
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mkdir script
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cd script
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elm init
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elm install dillonkearns/elm-pages-v3-beta
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```
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Now we can write our Script Module. Create a file called `script/src/Hello.elm` that exposes a top-level value `run` of type `Script`.
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```elm
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module Hello exposing (run)
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import Pages.Script as Script exposing (Script)
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run : Script
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run =
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Script.withoutCliOptions
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(Script.log "Hello from elm-pages Scripts!")
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```
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Now we can run our `Hello.elm` Script Module from the command line:
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```shell
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npx elm-pages run script/src/Hello.elm
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# Hello from elm-pages Scripts!
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```
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As a shorthand, you can run scripts from the folder `./script/src/` by passing in the Elm module name (without the `.elm` extension) instead of the file path of the module name.
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```python
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npx elm-pages run Hello
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# Hello from elm-pages Scripts!
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```
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## Script Folder
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The `script/` folder is a regular Elm project folder. That means it will need to have an `elm.json` file, and an Elm module in the `src/` folder (or whichever `source-directories` path you define in your `elm.json`). It will also need `elm-pages` installed as a dependency.
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If you use [`BackendTask.Custom.run`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/BackendTask-Custom#run) from your Script, it will use the `custom-backend-task.ts` (or `.js`) file defined in your `script/` project folder.
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The name `script/` is a convention and is not required. You can execute the `elm-pages run` command on any file as long as it is part of an Elm project (in its `source-directories`), and that project has `elm-pages` installed as a dependency.
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## Running Scripts from Different Directories
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You can pass any absolute or relative path to an `elm-pages` Script module to the `elm-pages run` command. It will find the `elm.json` for the Script module automatically.
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```shell
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elm-pages run ~/my-projects/elm-pages-scripts/src/Hello.elm
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```
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## Adding Command Line Options
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Scripts can define command line options with a config from [`dillonkearns/elm-cli-options-parser`](https://package.elm-lang.org/packages/dillonkearns/elm-cli-options-parser/latest/).
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Here is an example that takes optional keyword arguments `--username` and `--repo`.
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```elm
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module Stars exposing (run)
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import BackendTask exposing (BackendTask)
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import BackendTask.Http
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import Cli.Option as Option
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import Cli.OptionsParser as OptionsParser
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import Cli.Program as Program
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import Json.Decode as Decode
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import Pages.Script as Script exposing (Script)
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run : Script
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run =
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Script.withCliOptions program
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(\{ username, repo } ->
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BackendTask.Http.getJson
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("https://api.github.com/repos/dillonkearns/" ++ repo)
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(Decode.field "stargazers_count" Decode.int)
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|> BackendTask.andThen
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(\stars ->
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Script.log ("🤩" ++ (String.fromInt stars))
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)
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|> BackendTask.allowFatal
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)
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type alias CliOptions =
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{ username : String
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, repo : String
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}
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program : Program.Config CliOptions
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program =
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Program.config
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|> Program.add
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(OptionsParser.build CliOptions
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|> OptionsParser.with
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(Option.optionalKeywordArg "username"
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|> Option.withDefault "dillonkearns")
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|> OptionsParser.with
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(Option.optionalKeywordArg "repo"
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|> Option.withDefault "elm-pages")
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)
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```
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Our `Cli.Program.Config` that we defined automatically gives us a `--help` option.
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```
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npx elm-pages run script/src/Stars.elm --help
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# elm-pages run Stars [--username <username>] [--repo <repo>]
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```
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Now let's run our script with some options.
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```shell
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npx elm-pages run script/src/Stars.elm --repo elm-graphql
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# 🤩 757
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```
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Now let's try running it with an invalid option.
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```shell
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run script/src/Stars.elm --user elm --name json
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# -- Invalid CLI arguments ---------------
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# The `--name` flag was not found. Maybe it was one of these typos?
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#
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# `--name` <> `--username`
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```
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## `FatalError`'s
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If the `BackendTask` in your script resolves to a [`FatalError`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/FatalError), the script will print the error message and exit with a non-zero exit code. As with any `BackendTask`, if you want to ensure that your script will not encounter a `FatalError`, you can ensure that you have handled every possible error by using a value with the type `BackendTask Never ()`.
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## Scaffolding a Route Module
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The `elm-pages` package includes some modules that help you generate Elm files.
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`elm-pages run` will automatically run `elm-codegen install` for you if you have a `./codegen/` folder next to your `./script/` project.
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[`elm-codegen`](https://github.com/mdgriffith/elm-codegen) is a project that helps you write Elm code that generate Elm code. In our case, we are going to generate some scaffolding to help us add new Routes to our `elm-pages` app.
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`elm-codegen` helps ensure that you are generating valid code by generating functions that mirror the code they will generate, giving you an extra layer of type safety. `elm-pages` Scripts are general-purpose, so you can do what you want with them, but there are some built-in helpers to make it easy for you to scaffold code for your `elm-pages` app that work well with `elm-codegen`.
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Take a look at [the `AddRoute.elm` Script from the `elm-pages` starter repo](https://github.com/dillonkearns/elm-pages-3-alpha-starter/blob/main/script/src/AddRoute.elm). This Script is a great starting point for customizing your own scaffolding Scripts for your project. It is designed to lock in the essential details for defining a Route Module and a Form, while leaving the rest up to you to customize in the script. See [`Scaffold.Route`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/Scaffold-Route) and [`Scaffold.Form`](https://package.elm-lang.org/packages/dillonkearns/elm-pages-v3-beta/latest/Scaffold-Route) in the docs.
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## Compiling Scripts to an Executable JavaScript File
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You can compile your script (options parsing and all) to a single minified and optimized JavaScript file that you can run in any NodeJS environment.
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```shell
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elm-pages bundle-script script/src/Stars.elm
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```
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