使用haskell的collat​​z-list实现 [英] collatz-list implementation using haskell

问题描述

我正在尝试使用Haskel实现collat​​z-list: 这是我的代码:

I am trying to implement collatz-list using Haskel: Here's my code:

collatz n
     | mod n 2 == 0 = div n 2
     | otherwise =  3 * n + 1


collatzList n
        | n < 1 = error "Cannot have negative number"
collatzList
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)

我收到的错误消息是: 解析输入"collat​​zList"上的错误 [1 of 1]编译Main(exer.hs，已解释) 失败，模块已加载:无.

The error message I am getting is this: parse error on input `collatzList' [1 of 1] Compiling Main ( exer.hs, interpreted ) Failed, modules loaded: none.

有人可以告诉我为什么收到此消息吗?

Can anyone tell me why I am getting this message?

推荐答案

我遇到了其他错误(使用GHC 7.4.1):

I get different errors (using GHC 7.4.1):

> :load "/tmp/C.hs"
[1 of 1] Compiling Main             ( /tmp/C.hs, interpreted )

/tmp/C.hs:9:11: Not in scope: `n'

/tmp/C.hs:9:21: Not in scope: `n'

/tmp/C.hs:10:23: Not in scope: `n'

/tmp/C.hs:10:46: Not in scope: `n'
Failed, modules loaded: none.

这是因为您忘记了collatzList的第二个方程式中的n自变量.您可以添加此参数

This is because you forgot the n argument in your second equation for collatzList. You can either add this argument

collatzList n
        | n < 1 = error "Cannot have negative number"
collatzList n -- the n was missing here
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)

或者，由于左侧现在相同，因此您可以简单地将其与第一个连接起来:

or, since the left hand sides are now the same, you can simply join it with the first one:

collatzList n
        | n < 1 = error "Cannot have negative number"
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)

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