GHCi 中特殊情况的函数定义 [英] Function definition by special cases in GHCi

问题描述

来自 Haskell 教程:

From a Haskell tutorial:

我们可以根据情况编写整数函数.

We can write functions on integers by cases.

-- Compute the sum of the integers from 1 to n.
sumtorial :: Integer -> Integer
sumtorial 0 = 0
sumtorial n = n + sumtorial (n-1)

但是，当我尝试它时会发生以下情况:

However, here's what happens when I try it:

$ ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> foo 0 = print 999
Prelude> foo n = print n
Prelude> foo 0
0

我错过了什么?

推荐答案

要在 GHCi 中完全按照原样使用这些定义(即在单独的行中使用多个方程或类型签名)，您需要在 GHCi 中使用多行输入通过:{ 和 :} 分隔符:

To use those definition in GHCi exactly as they were written (i.e. with multiple equations or type signatures in separate lines), you need to use multiline input in GHCi through the :{ and :} delimiters:

GHCi> :{
GHCi| foo 0 = print 999
GHCi| foo n = print n
GHCi| :}
GHCi> foo 0
999

另一种选择是使用 +m 选项在会话的其余部分启用多行输入.但是，在这种情况下，您还需要一个显式的 let，因为没有它 GHCi 不会确定您要继续定义:

One alternative would be turing on multiline input for the rest of your session with the +m option. In this case, though, you also need an explicit let, as without it GHCi won't figure out you want to continue the definition:

GHCi> :set +m
GHCi> let foo 0 = print 999
GHCi|     foo n = print n
GHCi| 
GHCi> foo 0
999

(您可以使用 :unset +m 关闭 +m.)

(You can turn +m off with :unset +m.)

另一种可能性是完全避免换行，并使用显式大括号和分号:

Yet another possibility is eschewing line breaks altogether, and using explicit braces and semicolons:

GHCi> foo 0 = print 999; foo n = print n
GHCi> foo 0
999

在多行输入选项之间，我个人更喜欢 :{ 和 :} 而不是 +m，因为它们对我通常如何表述我的定义，如果我从其他地方粘贴代码，更有可能立即起作用.

Between the multiline input options, I personally prefer :{ and :} over +m, as they require less changes with respect to how I usually phrase my definitions, and are more likely to work straight away if I paste code from somewhere else.

至于为什么您的输入方式不起作用，这是因为，除非您使用多行输入，否则在单独的 GHCi 行中绑定到相同名称会相互遮蔽:

As for why your way of entering it didn't work, it was because, unless you use multiline input, bindings to the same name in separate GHCi lines shadow each other:

GHCi> x = 3
GHCi> x = 4
GHCi> x
4

如果我们注意到我们从 let 表达式链中得到相同的行为，这似乎就不那么令人惊讶了:

This seems less surprising if we note that we get the same behaviour from a chain of let-expressions:

GHCi> let x = 3 in let x = 4 in x
4

这篇关于GHCi 中特殊情况的函数定义的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！