如何在跨多行的ghci中定义一个函数？

问题描述

  
 
我试图定义任何跨越ghci多行的简单函数，让abs n | n> = 0 = n 
 |否则= -n 
  

到目前为止，我已经在第一行之后按Enter键：

  Prelude>让abs n | n> = 0 = n 
前奏> |否则= -n 
< interactive>：1：0：输入分析错误'|'
  

我也曾尝试使用：{和：} 命令，但我并没有走得太远：

  Prelude> ：{
未知命令'：{'
 use：？求助。 
  

我在Linux上使用GHC Interactive版本6.6来测试Haskell 98，我错过了什么？

  let abs n | n> = 0 = n |否则= -n 
  

如果你想用你的函数写出多个模式与参数匹配的定义，像这样：

pre code $ fact $ 0 $ 1
fact n = n * fact（n-1）

然后您可以使用分号分隔定义

  let {fact 0 = 1;事实n = n * fact（n-1）} 
  

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

let abs n | n >= 0 = n
          | otherwise = -n

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

Prelude> let abs n | n >= 0 = n
Prelude>           | otherwise = -n
<interactive>:1:0: parse error on input `|'

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

Prelude> :{
unknown command ':{'
use :? for help.

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

解决方案

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

let abs n | n >= 0 = n | otherwise = -n

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

fact 0 = 1
fact n = n * fact (n-1)

then you would use braces with semicolons separating the definitions

let { fact 0 = 1 ; fact n = n * fact (n-1) }

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