blocks|key|1033331|text|看起来:{和:}是一个非常新的特性。您可能需要升级GHC。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1033332|编辑:已确认，请参阅http://www.haskell.org/ghc/docs/6.8.2/html/users_guide/release-6-8-2.html|1033333|entityMap|0|LINK|mutability|MUTABLE|url|http://www.haskell.org/ghc/docs/6.8.2/html/users_guide/release-6-8-2.html^0|3|2|6|2|0|A|21|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@]|D|@$9|V|A|W|1|X]]|E|$]]|$1|H|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

It looks like <code>:{</code> and <code>:}</code> are a pretty new feature. You may need to upgrade GHC.

Edit: confirmed, see <a href="http://www.haskell.org/ghc/docs/6.8.2/html/users_guide/release-6-8-2.html" rel="noreferrer">http://www.haskell.org/ghc/docs/6.8.2/html/users_guide/release-6-8-2.html</a>

blocks|key|1248497|text|对于卫士(如您的示例)，您只需将它们放在一行中，它就可以工作(卫士不关心间距)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1248498|let+abs+n+%7C+n+>=+0+=+n+%7C+otherwise+=+-n|code-block|syntax|javascript|1248499|如果您想要编写具有多个定义的函数，这些定义的模式与参数匹配，如下所示：|1248500|fact+0+=+1
fact+n+=+n+*+fact+(n-1)|1248501|然后，您可以使用大括号和分号分隔定义|1248502|let+{+fact+0+=+1+;+fact+n+=+n+*+fact+(n-1)+}|1248503|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

For guards (like your example), you can just put them all on one line and it works (guards do not care about spacing)

<pre><code>let abs n | n &gt;= 0 = n | otherwise = -n
</code></pre>

If you wanted to write your function with multiple definitions that pattern match on the arguments, like this:

<pre><code>fact 0 = 1
fact n = n * fact (n-1)
</code></pre>

Then you would use braces with semicolons separating the definitions

<pre><code>let { fact 0 = 1 ; fact n = n * fact (n-1) }
</code></pre>

blocks|key|1248551|text|Dan是正确的，但是:{和:}必须分别出现在各自的行中：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1248552|>+:{+
>+let+foo+a+b+=+a+%2B
>+++++++++++b
>+:}
>+:t+foo
foo+::+(Num+a)+=>+a+->+a+->+a|code-block|syntax|javascript|1248553|这也与布局规则交互，所以当使用do-notation时，显式地使用大括号和分号可能更容易。例如，此定义失败：|1248554|>+:{
%7C+let+prRev+=+do
%7C+++inp+<-+getLine
%7C+++putStrLn+$+reverse+inp
%7C+:}
<interactive>:1:18:
++++The+last+statement+in+a+'do'+construct+must+be+an+expression|1248555|但当添加大括号和分号时，它会起作用：|1248556|>+:{
%7C+let+prRev+=+do+{
%7C+++inp+<-+getLine;
%7C+++putStrLn+$+reverse+inp;
%7C+}
%7C+:}
>+:t+prRev
prRev+::+IO+()|1248557|只有在从文件中粘贴定义时，这才是真正重要的，因为缩进可能会更改。|1248558|entityMap^0|A|2|D|2|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|16|8|@]|D|@]|E|$]]|$1|U|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|V|$]]

Dan is correct, but <code>:{</code> and <code>:}</code> must each appear on their own line:

<pre><code>&gt; :{ 
&gt; let foo a b = a +
&gt; b
&gt; :}
&gt; :t foo
foo :: (Num a) =&gt; a -&gt; a -&gt; a
</code></pre>

This also interacts with the layout rule, so when using do-notation it might be easier to use braces and semi-colons explicitly. For example, this definition fails:

<pre><code>&gt; :{
| let prRev = do
| inp &lt;- getLine
| putStrLn $ reverse inp
| :}
&lt;interactive&gt;:1:18:
 The last statement in a 'do' construct must be an expression
</code></pre>

But it works when braces and semi-colons are added:

<pre><code>&gt; :{
| let prRev = do {
| inp &lt;- getLine;
| putStrLn $ reverse inp;
| }
| :}
&gt; :t prRev
prRev :: IO ()
</code></pre>

This will only really matter when pasting definitions from a file, where indentation might change.

blocks|key|1033543|text|GHCi现在有一个多行输入模式，通过:set+%2Bm启用。例如，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1033544|Prelude>+:set+%2Bm
Prelude>+let+fac+0+=+1
Prelude%7C+++++fac+n+=+n+*+fac+(n-1)
Prelude%7C
Prelude>+fac+10
3628800|code-block|syntax|javascript|1033545|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

GHCi now has a multiline-input mode, enabled with :set +m. For example,

<pre><code>Prelude&gt; :set +m
Prelude&gt; let fac 0 = 1
Prelude| fac n = n * fac (n-1)
Prelude|
Prelude&gt; fac 10
3628800
</code></pre>

blocks|key|1033574|text|我在macOS+Catalina10.15.2上使用的是GHCi+8.2.1版。下面是我如何将函数类型声明和防护放在一起。请注意，左侧的竖线用于GHCi多行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1033575|λ:+let+abs'+::+(Num+a,+Ord+a)+=>+a+->+a
+%7C+++++abs'+n+%7C+n+>=+0+=+n+%7C+otherwise+=+-n
+%7C+
λ:+abs'+7
7
λ:+abs'+(-7)
7|code-block|syntax|javascript|1033576|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I'm using GHCi, version 8.2.1 on macOS Catalina 10.15.2. The following is how I put both function type declaration and guards together. Note the vertical bars on the left are for GHCi multiple lines. 

<pre><code>λ: let abs' :: (Num a, Ord a) =&gt; a -&gt; a
 | abs' n | n &gt;= 0 = n | otherwise = -n
 | 
λ: abs' 7
7
λ: abs' (-7)
7
</code></pre>

blocks|key|1033636|text|它看起来像是一次粘贴两行，或者对每个新行使用control+enter组合键，至少在https://repl.it/languages/haskell中是这样的。你会在第二行的开头看到两个点。或者将其放入一个文件中，然后:加载该文件(:l+main)。为什么abs对负数不起作用？哦，你必须用括号把数字括起来。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1033637|+++let+abs+n+%7C+n+>=+0+=+n+
..+++++++++++%7C+otherwise+=+-n|code-block|syntax|javascript|1033638|+++abs+(-1)|1033639|entityMap|0|LINK|mutability|MUTABLE|url|https://repl.it/languages/haskell^0|16|X|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|F|7|X|8|@]|9|@]|C|$G|H]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

It looks like pasting both lines at once or using control-enter for each new line keeps it all together, at least at <a href="https://repl.it/languages/haskell" rel="nofollow noreferrer">https://repl.it/languages/haskell</a>. You'll see 2 dots in the beginning of the second line. Or put it in a file and :load the file (:l main). How come abs doesn't work with negative numbers? Oh you have to put parentheses around the number.
<pre><code> let abs n | n &gt;= 0 = n 
.. | otherwise = -n
</code></pre>
<pre><code> abs (-1)
</code></pre>

I'm trying to define any simple function that spans multiple lines in ghci, take the following as an example:

<pre><code>let abs n | n &gt;= 0 = n
 | otherwise = -n
</code></pre>

So far I've tried pressing Enter after the first line:

<pre><code>Prelude&gt; let abs n | n &gt;= 0 = n
Prelude&gt; | otherwise = -n
&lt;interactive&gt;:1:0: parse error on input `|'
</code></pre>

I've also attempted to use the <code>:{</code> and <code>:}</code> commands but I don't get far:

<pre><code>Prelude&gt; :{
unknown command ':{'
use :? for help.
</code></pre>

I'm using GHC Interactive version 6.6 for Haskell 98 on Linux, what am I missing?

How to define a function in ghci across multiple lines?

Linux

我正在尝试在ghci中定义任何跨越多行的简单函数，下面是一个例子：let abs n | n >= 0 = n          | otherwise = -n到目前为止，我已经尝试在第一行之后按Enter键：Prelude> let abs n | n >= 0 = nPrelude>           | ot...

问如何在ghci中跨多行定义一个函数？
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