为什么这个代码返回空列表或失败？哈斯克尔 [英] Why this code return empty list or fail? Haskell

问题描述

primes :: [Int]
primes = sieve [2..]

sieve :: [Int] -> [Int]
sieve (p:xs)= p:sieve[x|x<-xs,x `mod` p /= 0]

f :: Int->Int
f n = head [x|x<-[0,(product (filter (<n) primes))..],x/=0,sum (map (x `mod`) [1..n]) == 0]

---

加载到GHCi
并键入f 20
失败...... plz help me

---

load into GHCi and type "f 20" fail...... plz help me

推荐答案

问题是用过滤器（< n）素数。由于素数是一个无穷大非常长的 ¹ 列表，所以需要很长时间终止，因为它不知道那么（< n）最终会返回 False 继续检查整个列表。

The problem is with filter (< n) primes. Since primes is an infinite very long¹ list, this takes a long time to terminate since it doesn't know that (< n) will eventually return False for all primes past a certain point in the list, so it has to keep checking the entire list.

> filter (< 20) primes
[2,3,5,7,11,13,17,19^CInterrupted.

使用 takeWhile（< n）素数取而代之。

Use takeWhile (< n) primes instead.

> takeWhile (< 20) primes
[2,3,5,7,11,13,17,19]

_{¹ 由于 [Int] 类型，它是有限的。如果它是 [Integer] ，它将是无限的。}

_{¹ It's finite because of the type [Int]. If it was [Integer], it would be infinite.}

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