async registryNameToUrl(buildpack: string): Promise<string> {
    if (validUrl.isWebUri(buildpack)) {
      return buildpack
    }

    Result.match({
      Ok: _ => {},
      Err: err => {
        cli.error(`Could not find the buildpack: ${buildpack}. ${err}`, {exit: 1})
      },
    }, BuildpackRegistry.isValidBuildpackSlug(buildpack))

    try {
      let response = await this.registry.buildpackExists(buildpack)
      let body = await response.json()
      return body.blob_url
    } catch (err) {
      if (err.statusCode === 404) {
        cli.error(`${buildpack} is not in the buildpack registry.`, {exit: 1})
      } else if (err.statusCode) {
        cli.error(`${err.statusCode}: ${err.message}`, {exit: 1})
      } else {
        cli.error(err.message, {exit: 1})
      }
    }

    return ''
  }

 validateIndexInRange(buildpacks: BuildpackResponse[], index: number) {
   if (index < 0 || index > buildpacks.length) {
     if (buildpacks.length === 1) {
       cli.error('Invalid index. Only valid value is 1.', {exit: 1})
     } else {
       cli.error(`Invalid index. Please choose a value between 1 and ${buildpacks.length}`, {exit: 1})
     }
   }
 }

 displayUpdate(app: string, remote: string, buildpacks: BuildpackResponse[], action: 'added' | 'set' | 'removed') {
   if (buildpacks.length === 1) {
     cli.log(`Buildpack ${action}. Next release on ${app} will use ${this.registryUrlToName(buildpacks[0].buildpack.url)}.`)
     cli.log(`Run ${color.magenta(push(remote))} to create a new release using this buildpack.`)
   } else {
     cli.log(`Buildpack ${action}. Next release on ${app} will use:`)
     this.display(buildpacks, '  ')
     cli.log(`Run ${color.magenta(push(remote))} to create a new release using these buildpacks.`)
   }
 }

  async clear(app: string, command: 'clear' | 'remove', action: 'cleared' | 'removed') {
    await this.put(app, [])

    let configVars: any = await this.heroku.get(`/apps/${app}/config-vars`)
    let message = `Buildpack${command === 'clear' ? 's' : ''} ${action}.`
    if (configVars.body.BUILDPACK_URL) {
      cli.log(message)
      cli.warn('The BUILDPACK_URL config var is still set and will be used for the next release')
    } else if (configVars.body.LANGUAGE_PACK_URL) {
      cli.log(message)
      cli.warn('The LANGUAGE_PACK_URL config var is still set and will be used for the next release')
    } else {
      cli.log(`${message} Next release on ${app} will detect buildpacks normally.`)
    }
  }

 display(buildpacks: BuildpackResponse[], indent: string) {
   if (buildpacks.length === 1) {
     cli.log(this.registryUrlToName(buildpacks[0].buildpack.url, true))
   } else {
     buildpacks.forEach((b, i) => {
       cli.log(`${indent}${i + 1}. ${this.registryUrlToName(b.buildpack.url, true)}`)
     })
   }
 }

 async validateUrlNotSet(buildpacks: BuildpackResponse[], buildpack: string) {
   if (await this.findUrl(buildpacks, buildpack) !== -1) {
     cli.error(`The buildpack ${buildpack} is already set on your app.`, {exit: 1})
   }
 }

 Err: err => {
   cli.error(`Could not find the buildpack: ${buildpack}. ${err}`, {exit: 1})
 },

 validateIndex(index: number) {
   if (isNaN(index) || index <= 0) {
     cli.error('Invalid index. Must be greater than 0.', {exit: 1})
   }
 }