Generating typed definitions from environment variables

An example of how to use embeds/env to generate constants of other types, like integers, boolean flags, or custom enums.

You can check out the full example project here.

Running

Run the project, providing some environment variables:

BUILD_ENV=production \
BUILD_KNOWN_BUGS=9001.000000004 \
BUILD_ENABLE_AWESOMENESS=YES \
BUILD_REVISION=16 \
gleam run

Full Example Source

NOTE: In a real project, you would add a seperate build path dev dependency. This avoids issues when regenerating files that your project already depends on.

import embeds/env
import embeds/generate
import gleam/io
import gleam/option

pub fn main() {
  let result =
    env.generate(env.Options(
      module: "build_env",
      prefix: "BUILD_",
      regex: option.None,
      parse: env.typed_parse([
        #("BUILD_ENV", environment),
        #("BUILD_ENABLE_AWESOMENESS", env.bool),
        // needs to be float because otherwise the login email doesn't work
        #("BUILD_KNOWN_BUGS", env.float),
        #("BUILD_REVISION", env.string),
      ]),
      // we need to include the environment type we want to use somehow -
      // right here we just put it onto the top of the generated file.
      // an alternative might be to put this type into a `shared` project.
      header: "
        pub type Environment {
          Development
          Testing
          Production
        }
      ",
    ))

  case result {
    Ok(Nil) -> Nil
    Error(errs) -> io.println_error(generate.describe_errors(errs))
  }
}

// a custom environment parser
fn environment(value: String) -> generate.Code {
  // requires us to set the header above to include the type!
  case value {
    "development" -> generate.Constant("Development")
    "production" -> generate.Constant("Production")
    "testing" -> generate.Constant("Testing")
    _ -> generate.GenerateError("Invalid environment: " <> value)
  }
}