CODEWITHSUNDEEP
listhaskellprimes
Netherar
Netherar

Reputation: 15

Modify haskell function to run correctly

I have the following haskell code:

a (b : bs) = b : [c | c <- (a bs), c `rem` b /= 0]

Can someone explain what this code does? Running a as

a [3,5..42]

returns 

  Prelude> a [3,5..42]
[3,5,7,11,13,17,19,23,29,31,37,41*** Exception: <interactive>:71:1-46: Non-exhaustive patterns in function a

From what i can see, the function works like the Sieve of Eratosthenes. The function considers b as a prime number and filters out the multiples of b. But i'm not really sure how. On top of that, the function throws this exception.

Upvotes: 0

Views: 80

Answers (1)

Professor Flitwick
Professor Flitwick

Reputation: 71

What you have here is a recursive function: You are calling a bs in your definition. Eventually bs will be the empty list, and at that point you get an exception. You can, for example, add the following line to your code:

a [] = []

Then the output will become:

[3,5,7,11,13,17,19,23,29,31,37,41]

As for what this function does, it returns every element in the list which is not a multiple of any previous element in the list. If you give it a list [2..x] where x is any integer, this is the same thing as a list of all prime numbers from 2 to x.

Another way of getting a list of primes is the one you've found: You start from 3 and make the Haskell list comprehension skip any multiples of 2.

Upvotes: 6

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