List.filter : (a -> Bool) -> List a -> List a是一个高阶函数，它将一个参数函数从任何值转换为布尔值，并将该函数应用于给定列表的每个元素，仅保留该函数为其返回的那些元素True。List.filter以第一个参数为函数的函数通常称为谓词。
import String catStory : List String catStory = ["a", "crazy", "cat", "walked", "into", "a", "bar"] -- Any word with more than 3 characters is so long! isLongWord : String -> Bool isLongWord string = String.lengthstring > 3 longWordsFromCatStory : List String longWordsFromCatStory = List.filterisLongWord catStory
在中对此进行评估elm-repl：
> longWordsFromCatStory ["crazy", "walked", "into"] : List String > >List.filter(String.startsWith "w") longWordsFromCatStory ["walked"] : List String