yesod-recaptcha2

hackage

import Yesod.ReCaptcha2

instance YesodReCaptcha App where
    reCaptchaSiteKey = pure "foo"
    reCaptchaSecretKey = pure "bar"
    reCaptchaLanguage = pure Nothing

Append to applicative form

buildForm :: Form MyForm
buildForm = renderDivs $ MyForm
  <$> areq textField "foo" Nothing
  <* reCaptcha