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lambda :: [String] -> Parser ()
lambda variables =
  do
    char '('
    choice
      [ do
        string "abs "
        name <- many $ charBy (`elem` ['a'..'z'])
        guard $ notElem name variables
        char ' '
        lambda (name : variables)
      , do
        string "var "
        choice (map string variables)
        return ()
      , do
        string "app "
        lambda variables
        char ' '
        lambda variables
      ]
    char ')'
    return ()

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lambda :: [String] -> Parser ()
lambda variables =
  do
    char '('
    choice
      [ do
        string "abs "
        name <- some $ charBy (`elem` ['a'..'z'])
        guard $ notElem name variables
        char ' '
        lambda (name : variables)
      , do
        string "var "
        choice (map string variables)
        return ()
      , do
        string "app "
        lambda variables
        char ' '
        lambda variables
      ]
    char ')'
    return ()

Try it online!