blocks|key|601796|text|我只是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|601797|getNotes+root+=+[root+..+maxBound]+%2B%2B+init+[minBound+..+root]|code-block|syntax|javascript|601798|但我看得出来你更喜欢循环的方法。怎么样|601799|getNotes+root+=+map+snd+.+take+12+$+[(0,root)+..+]|601800|...sadly，这实际上是行不通的:它需要一个(Enum+a,+Enum+b,+Bounded+b)+=>+Enum+(a,b)实例，由于某种原因没有定义它，至少在序曲中没有定义。|offset|length|style|CODE|601801|或者，您可以使用root的索引。|601802|getNotes+root+=+take+12+.+drop+(fromEnum+root)+$+cycle+[minBound+..+maxBound]|601803|entityMap^0|0|0|0|0|O|15|0|8|4|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@$M|11|N|12|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@$M|14|N|15|O|P]]|9|@]|A|$]]|$1|S|3|T|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|U|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|V|$]]

I'd just

<pre><code>getNotes root = [root .. maxBound] ++ init [minBound .. root]
</code></pre>

but I can see how you prefer the cyclic approach. How about

<pre><code>getNotes root = map snd . take 12 $ [(0,root) .. ]
</code></pre>

...sadly, that doesn't in fact work: it would need a <code>(Enum a, Enum b, Bounded b) =&gt; Enum (a,b)</code> instance, which for some reason isn't defined, at least not in the prelude.

Alternatively, you can use the index of <code>root</code>:

<pre><code>getNotes root = take 12 . drop (fromEnum root) $ cycle [minBound .. maxBound]
</code></pre>

blocks|key|2383349|text|基于@leftaroundabout的第二个想法，这是一个有用的版本--以防您好奇并想玩它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2383350|{-#+LANGUAGE+ScopedTypeVariables+#-}

module+Stackoverflow+where

data+Note+=+C+%7C+CsDb+%7C+D+%7C+DsEb+%7C+E+%7C+F+%7C+FsGb+%7C+G+%7C+GsAb+%7C+A+%7C+AsBb+%7C+B
++++++++++deriving+(Show,+Enum,+Bounded)

instance+(Enum+a,+Enum+b,+Bounded+b)+=>+Enum+(a,b)+where
++toEnum+i+=
++++let+(d,m)+=+i+`divMod`+(fromEnum+(maxBound+::+b)+%2B+1)
++++in+(toEnum+d,+toEnum+m)
++fromEnum+(a,+b)+=+fromEnum+a+*+(fromEnum+(maxBound+::+b)+%2B+1)+%2B+fromEnum+b

getNotes+::+Note+->+[Note]
getNotes+root+=+map+snd+.+take+12+$+[(0,root)+..+]|code-block|syntax|javascript|2383351|示例：|2383352|λ>+getNotes+E
[E,F,FsGb,G,GsAb,A,AsBb,B,C,CsDb,D,DsEb]|2383353|PS:这个想法非常聪明--左撇子--所以男人们一定要给他很多选票;)|2383354|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Based on the second idea of @leftaroundabout this is a working version - just in case you are curious and want to play with it:

<pre><code>{-# LANGUAGE ScopedTypeVariables #-}

module Stackoverflow where

data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B
 deriving (Show, Enum, Bounded)

instance (Enum a, Enum b, Bounded b) =&gt; Enum (a,b) where
 toEnum i =
 let (d,m) = i `divMod` (fromEnum (maxBound :: b) + 1)
 in (toEnum d, toEnum m)
 fromEnum (a, b) = fromEnum a * (fromEnum (maxBound :: b) + 1) + fromEnum b

getNotes :: Note -&gt; [Note]
getNotes root = map snd . take 12 $ [(0,root) .. ]
</code></pre>

example:

<pre><code>λ&gt; getNotes E
[E,F,FsGb,G,GsAb,A,AsBb,B,C,CsDb,D,DsEb]
</code></pre>

<hr>

PS: the idea is extremely smart @leftaroundabout &lt;- so guys make sure to give him lot's of upvotes ;)

blocks|key|2383367|text|你能做的最小的改变就是使用dropWhile|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2383368|getNotes+::+Note+->+[Note]
getNotes+root+=+take+12+.+dropWhile+(/=+root)+.+cycle+$+[minBound+..+maxBound]|code-block|syntax|javascript|2383369|entityMap^0|D|9|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

The smallest change you can make that works is to use <code>dropWhile</code>:

<pre><code>getNotes :: Note -&gt; [Note]
getNotes root = take 12 . dropWhile (/= root) . cycle $ [minBound .. maxBound]
</code></pre>

blocks|key|511244|text|怎么样|type|unstyled|depth|inlineStyleRanges|entityRanges|data|511245|getNotes+::+Note+->+[Note]
getNotes+root+=+ys+%2B%2B+xs+where+(xs,ys)+=+break+(==root)+[minBound..maxBound]|code-block|syntax|javascript|511246|？这与@leftaroundabout的第一个建议大致相同，它避免了init，但引起了一些等式比较：)|offset|length|style|CODE|511247|entityMap^0|0|0|Y|4|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

How about

<pre><code>getNotes :: Note -&gt; [Note]
getNotes root = ys ++ xs where (xs,ys) = break (==root) [minBound..maxBound]
</code></pre>

? This is more-or-less the same as @leftaroundabout's first suggestion, avoids the <code>init</code>, but incurs a number of equality comparisons :-)

Let's say I have a list of twelve musical notes (which have their own data type), and I want a function that returns a list of notes starting with a given note and looping around.

<pre><code>data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B deriving (Read, Eq, Ord, Enum, Bounded)
getNotes :: Note -&gt; [Note]
getNotes root = take 12 $ doSomething root $ cycle noteList
 where noteList :: [Note]
 noteList = [minBound..maxBound]
</code></pre>

such that

<pre><code>ghci&gt; getNotes E
[E, F, FsGb, G, GsAb, A, AsBb, B, C, CsDb, D, DsEb] 
</code></pre>

I can think of a few sloppy ways to do this, but it feels like there should be an obvious, very Haskellian way. Any recommendations?

Generating a sublist from cycle in Haskell

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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假设我有一个包含12个音符的列表(它们有自己的数据类型)，我想要一个函数，返回一个以给定音符开始并循环的音符列表。data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B deriving (Read, Eq, Ord, Enum,...

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