Include environment variables in lustre bundle

[dinkelspiel]

Feature request for including environment variables in lustre bundles. Useful for changing behaviors or values between development and production etc.

[andriishupta]

Hello, I wanted to get env variables to SPA(like API_URL) and found a discussion on Discord

looking that quite some time has passed since this ticket was created, I wonder how this PROPOSAL would fit Gleam's language design:

1. populate env variables

• create a new template in lustre_dev/priv/template for env variables
• in build.gleam inside do_app create a new populated file from cli.template("env.mjs")
  inside env.mjs, we would need to add an Object with all populated env values and some code(could be extended with get_or)

export function lustre_env() {
  // populated
  const vals = {
    ...
  }
  return {
    get(key) {
      return vals[key] ? Ok(vals[key]) : Error(null)
    },
    get_or() {}
  }
}

Inside this file we could automatically add LUSTRE_PUBLIC_* and additionally, populate flags from --env: gleam run -m lustre/dev build --minify --outdir=../server/priv/static --env=ADDITIONAL_ENV_VAR=true,ANOTHER=example

• add lustre_env() to entry-with-main.mjs inside main(..): context({ env })

2. lustre/context, simplified

pub type LustreEnv {
  LustreEnv(get: fn(String) -> Result(String, Nil))
}

pub type LustreContext { // in case you want to add something more than `env`
  LustreContext(env: LustreEnv)
}

3. usage

import lustre/context.{LustreContext, LustreEnv}

pub fn main(context: Option(LustreContext)) {
  let assert Some(context) = context
  let env = context.env
  let api_url_result = env.get("LUSTRE_PUBLIC_API_URL")
   ...
   
  let flags =
    dict.new()
    |> dict.insert("api_url", result.unwrap(api_url_result, "http://api:3030")

  let app = lustre.application(init, update, view)
  let assert Ok(_) = lustre.start(app, "#app", flags)

with this design, we won't use the generation of gleam files

ANOTHER option I was thinking about, which is worse, is to use esbuild --define=[...] that will replace env variables inline in the generated code
in Gleam it could also be called with something like lustre_env("LUSTRE_PUBLIC_API") that will be replaced in JS with the value