blocks|key|1290191|text|您希望在什么意义上组合这些monads？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1290192|f+::+Int+->+IO+(Maybe+Int)
f+x+=+do
++++putStrLn+"Hello+world!"
++++return+$+if+x+==+0+then+Nothing+else+Just+x|code-block|syntax|javascript|1290193|可以评估为：|1290194|[1+of+1]+Compiling+Main+++++++++++++(+maybe-io.hs,+interpreted+)
Ok,+modules+loaded:+Main.
*Main>+f+0
Hello+world!
Nothing
*Main>+f+3
Hello+world!
Just+3|1290195|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

In what sense do you want to combine the monads?

<pre><code>f :: Int -&gt; IO (Maybe Int)
f x = do
 putStrLn "Hello world!"
 return $ if x == 0 then Nothing else Just x
</code></pre>

Can be evaluated to:

<pre><code>[1 of 1] Compiling Main ( maybe-io.hs, interpreted )
Ok, modules loaded: Main.
*Main&gt; f 0
Hello world!
Nothing
*Main&gt; f 3
Hello world!
Just 3
</code></pre>

blocks|key|1504893|text|您并没有确切地说出希望如何组合IO和Maybe，但我假设您有许多返回IO+(Maybe+a)的函数，您希望轻松组合它们。基本上，您希望使用自己的Monad实例将IO+(Maybe+a)视为一个单独的类型：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1504894|newtype+IOMaybe+a+=+IOM+(IO+(Maybe+a))

--+"unpack"+a+value+of+the+new+type
runIOMaybe+::+IOMaybe+a+->+IO+(Maybe+a)
runIOMaybe+(IOM+a)+=+a

instance+Monad+IOMaybe+where
+++--+bind+operator
+++(IOM+ioa)+>>=+f+=+IOM+$+do
++++++a+<-+ioa
++++++case+a+of
+++++++++Nothing+->+return+Nothing
+++++++++Just+v++->+runIOMaybe+(f+v)

+++--+return
+++return+a+=+IOM+(return+(Just+a))

--+maybe+also+some+convenience+functions
returnIO+::+IO+a+->+IOMaybe+a
returnIO+ioa+=+IOM+$+do
+++v+<-+ioa
+++return+(Just+v)

returnMaybe+::+Maybe+a+->+IOMaybe+a
returnMaybe+ma+=+IOM+(return+ma)|code-block|syntax|javascript|1504895|这样，您就可以使用do-Notation组合返回IO+(Maybe+a)、IO+a或Maybe+a的函数|1504896|f1+::+Int+->+IO+(Maybe+Int)
f1+0+=+return+Nothing
f1+a+=+return+(Just+a)

main+=+runIOMaybe+$+do
+++returnIO+$+putStrLn+"Hello"
+++a+<-+returnMaybe+$+Just+2
+++IOM+$+f1+a
+++return+()|1504897|通常，像这样组合和修改monad的东西被称为monad+transformer，GHC附带了一个package，其中包括常见情况下的monad转换器。这个monad+transformer库中是否有适合您的场景的内容取决于您想要如何准确地组合Maybe和IO。|1504898|entityMap|0|LINK|mutability|MUTABLE|url|http://haskell.org/haskellwiki/Monad_Transformers_Explained|1|http://hackage.haskell.org/package/mtl/^0|F|2|I|5|Y|C|20|5|28|C|0|0|9|2|O|C|11|4|16|7|0|0|M|H|0|1C|7|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1L|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1M|8|@]|D|@$9|1N|A|1O|1|1P]|$9|1Q|A|1R|1|1S]]|E|$]]|$1|Q|3|-4|5|6|7|1T|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]|Y|$5|T|U|V|E|$W|Z]]]]

You don't exactly say how you want to combine <code>IO</code> and <code>Maybe</code>, but I assume you have many functions that return <code>IO (Maybe a)</code> that you want to combine easily. Basically you want to treat <code>IO (Maybe a)</code> as a separate type with it's own <code>Monad</code> instance:

<pre><code>newtype IOMaybe a = IOM (IO (Maybe a))

-- "unpack" a value of the new type
runIOMaybe :: IOMaybe a -&gt; IO (Maybe a)
runIOMaybe (IOM a) = a

instance Monad IOMaybe where
 -- bind operator
 (IOM ioa) &gt;&gt;= f = IOM $ do
 a &lt;- ioa
 case a of
 Nothing -&gt; return Nothing
 Just v -&gt; runIOMaybe (f v)

 -- return
 return a = IOM (return (Just a))

-- maybe also some convenience functions
returnIO :: IO a -&gt; IOMaybe a
returnIO ioa = IOM $ do
 v &lt;- ioa
 return (Just v)

returnMaybe :: Maybe a -&gt; IOMaybe a
returnMaybe ma = IOM (return ma)
</code></pre>

With this you can use the <code>do</code>-Notation to combine functions that return <code>IO (Maybe a)</code>, <code>IO a</code> or <code>Maybe a</code>:

<pre><code>f1 :: Int -&gt; IO (Maybe Int)
f1 0 = return Nothing
f1 a = return (Just a)

main = runIOMaybe $ do
 returnIO $ putStrLn "Hello"
 a &lt;- returnMaybe $ Just 2
 IOM $ f1 a
 return ()
</code></pre>

Generally something that combines and modifies monads like this is called a <a href="http://haskell.org/haskellwiki/Monad_Transformers_Explained" rel="noreferrer">monad transformer</a>, and GHC comes with a <a href="http://hackage.haskell.org/package/mtl/" rel="noreferrer">package</a> that includes monad transformers for common cases. If there is something in this monad transformer library that fits your scenario depends on how exactly you want to combine Maybe and IO.

blocks|key|1290315|text|我假设您想要使用“可能单子”来提前终止(就像C中的break或return+)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1290316|在这种情况下，您应该使用MaybeT包(cabal+install+MaybeT)中的MaybeT。|1290317|main+=+do
++runMaybeT+.+forever+$+do
++++liftIO+$+putStrLn+"I+won't+stop+until+you+type+pretty+please"
++++line+<-+liftIO+getLine
++++when+("pretty+please"+==+line)+mzero
++return+()|code-block|syntax|javascript|1290318|MaybeT是可能monad的monad+transformer版本。|1290319|Monad转换器向其他Monad“添加功能”。|1290320|entityMap^0|P|5|V|6|0|K|K|17|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

I assume you want to use the Maybe monad for early termination (like <code>break</code> or <code>return</code> in C).

In that case you should use <code>MaybeT</code> from the MaybeT package (<code>cabal install MaybeT</code>).

<pre><code>main = do
 runMaybeT . forever $ do
 liftIO $ putStrLn "I won't stop until you type pretty please"
 line &lt;- liftIO getLine
 when ("pretty please" == line) mzero
 return ()
</code></pre>

MaybeT is a monad transformer version of the maybe monad.

Monad transformers "add functionality" to other monads.

Particularly, I need to be able to combine the CGI monad with the IO monad, but an example of how to combine the IO monad with the Maybe monad might be even better...

How do I combine monads in Haskell?

特别是，我需要能够组合CGI monad和IO monad，但如何组合IO monad和可能monad的示例可能会更好……

问如何在Haskell中组合monads？
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