一种可能的解决方案是：

import Control.Applicative
import Data.Foldable

data Structure = Structure String [Structure]
               deriving Show

lookFor :: String -> Structure -> Maybe Structure
lookFor a s@(Structure b xs)
   | a == b    = Just s
   | otherwise = asum (map (lookFor a) xs)

上面，map (lookFor a) xs在子结构中搜索a。这将产生一个[Maybe Structure]，我们使用asum来获取第一个Just _值，以便我们可以返回它。（如果没有找到这样的值，则asum返回Nothing。）
如果您不想从库中利用asum，那么定义它是一个很好的初学者练习。

作为递归结构，您需要设置一个基类。在你的例子中，当你用尽了列表中的所有结构时，就会发生这种情况。
作为@WillemVanOnsem，最好返回Maybe Structure，因为lookFor函数可能找不到Structure

import Data.List (elemIndex)
import Data.Maybe (fromJust, isJust)

data Structure = Structure String [Structure]

name :: Structure -> String
name (Structure a xs) = a

subStrcts :: Structure -> [String]
subStrcts (Structure a []) = []
subStrcts (Structure a xs) = [name x | x <- xs]

lookFor :: String -> Structure -> Maybe Structure
lookFor a (Structure b xs) 
    | null xs = Nothing
    | isJust maybeIndex = fmap (xs !!) maybeIndex
    | otherwise = lookFor a (Structure (name (head xs)) (tail xs))
    where maybeIndex = elemIndex a (subStrcts (Structure b xs))