Haskell 问题有标准库解决方案吗？
我想使用 Data.List.groupBy 根据 snd 元素的相等性对元组列表进行分组。
我可以这样做：
groupBy (\l r -> snd l == snd r) listOfTuples
但它让我觉得比较函数中有太多的样板——特别是因为如果我进行更复杂的比较，它可能会变得更加混乱。我想做类似的事情：
groupBy (comparing snd) listOfTuples
但是比较的类型签名是 comparing :: (Ord a) => (b→a)→ b-> b->排序，因此在此示例中无法编译。
我也可以这样做：
groupBy (\l r -> (comparing snd l r) == EQ) listOfTuples
但这并不比第一次尝试好。在我自己推出之前，是否有一个标准库解决方案可以解决这个问题？
I want to use Data.List.groupBy to group a list of tuples based on the equality of the snd element.
I could do this:
groupBy (\l r -> snd l == snd r) listOfTuples
But it strikes me as too much boilerplate in the comparison function -- especially because it could get a lot more messy if I were doing a more complicated comparison. I would like to do something like:
groupBy (comparing snd) listOfTuples
but the type signature of comparing is comparing :: (Ord a) => (b -> a) -> b -> b -> Ordering, so it doesn't compile in this example.
I could also do:
groupBy (\l r -> (comparing snd l r) == EQ) listOfTuples
But this is no better than the first try. Is there a standard-library solution to this problem, before I roll-my-own?
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我认为标准库中曾经有
equating = on (==)，尽管我现在似乎找不到它。I think there used to be
equating = on (==)in the standard libraries, though I can't seem to find it now.这是你想要的吗？
Is this what you want?