haskell - 第二个最近的列表项 Haskell

Question

我正在尝试获取 Haskell 列表中的第二项。我认为最好的方法是获取列表尾部的头部。

secondMostRecentChoice :: History -> Choice // a choice is just a Bool like Bake | NoBake
secondMostRecentChoice [] = "Not Enough History"
secondMostRecentChoice history = 
    if ( length history == 1 ) 
        then "Still Not Enough History"
    else if (length history >= 2)
        then (head [b | (head a,b) <- history]) //Here is the problem
        else "Not supposed to be here"
    else "Not supposed to be here"

但我得到以下信息:

Parse error in pattern: head
Possibly caused by a missing 'do'?

为什么我需要做 do或者这是一个错误的建议？

Answer 1

做到这一点的最短方法就是模式匹配它。

secondElem :: [a] -> Maybe a
secondElem (_:x:_) = Just x
secondElem _       = Nothing