Usage: `Spanish.PastTense.conjugate(subject, verb_on_infinitive)`

spanish.ex

defmodule Spanish.PastTense do
  alias Spanish.Participle
  import String, only: [capitalize: 1]

  @to_have %{"yo" => "he",
             "tú" => "has",
             "él" => "ha", # same for "ella"
             "nosotros" => "hemos",
             "ustedes" => "han"} # same for "ellos" and "ellas"
  @singular_exceptions ["ella", "eso"]
  @plural_exceptions ["ellos", "ellas"]

  def conjugate(subject, verb) when subject in @singular_exceptions,
    do: capitalize(subject) <> form_verb("él", verb)

  def conjugate(subject, verb) when subject in @plural_exceptions,
    do: capitalize(subject) <> form_verb("ustedes", verb)

  def conjugate(subject, verb),
    do: capitalize(subject) <> form_verb(subject, verb)

  defp form_verb(subject, verb),
    do: " " <> @to_have[subject] <> " " <> Participle.conjugate(verb)
end

defmodule Spanish.Participle do
  @rules %{"ar" => "ado",
           "ir" => "ido",
           "er" => "ido"}

  def conjugate(verb) do
    Enum.reduce(@rules, verb, fn({ending, new_ending}, verb)->
      # Check if `verb` ends with current `ending`
      # (Regex copied shamelessly from StackOverflow)
      if Regex.match?(~r/\b\p{L}*#{ending}\b/, verb),
        do: String.replace(verb, ending, new_ending), else: verb
    end)
  end
end