haskell - 实现 GHC.Exts 'the' 函数的 Maybe 版本

Question

GHC.Exts 导出函数 the :

the ensures that all the elements of the list are identical and then returns that unique element

此函数是部分函数，​​因为如果列表中的所有元素不相等，它会抛出错误。

如何实现返回 Maybe 的函数 theMay？

Answer 1

定义如下:

the :: Eq a => [a] -> a
the (x:xs)
  | all (x ==) xs = x
  | otherwise     = error "GHC.Exts.the: non-identical elements"
the []            = error "GHC.Exts.the: empty list"

据此我们可以直接推导出theMay:

theMay :: Eq a => [a] -> Maybe a
theMay (x:xs)
  | all (x ==) xs = Just x
  | otherwise     = Nothing
theMay []         = Nothing