# Sieve in Haskell

#### Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

1 | ```
exercism fetch haskell sieve
``` |

# Sieve

Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

The Sieve of Eratosthenes is a simple, ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite (i.e. not prime) the multiples of each prime, starting with the multiples of 2. It does not use any division or remainder operation.

Create your range, starting at two and continuing up to and including the given limit. (i.e. [2, limit])

The algorithm consists of repeating the following over and over:

- take the next available unmarked number in your list (it is prime)
- mark all the multiples of that number (they are not prime)

Repeat until you have processed each number in your range.

When the algorithm terminates, all the numbers in the list that have not been marked are prime.

The wikipedia article has a useful graphic that explains the algorithm: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

Notice that this is a very specific algorithm, and the tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes. A good first test is to check that you do not use division or remainder operations (div, /, mod or % depending on the language).

## Getting Started

For installation and learning resources, refer to the exercism help page.

## Running the tests

To run the test suite, execute the following command:

1 |
```
stack test
``` |

#### If you get an error message like this...

1 |
```
No .cabal file found in directory
``` |

You are probably running an old stack version and need to upgrade it.

#### Otherwise, if you get an error message like this...

1 2 |
No compiler found, expected minor version match with... Try running "stack setup" to install the correct GHC... |

Just do as it says and it will download and install the correct compiler version:

1 |
```
stack setup
``` |

## Running *GHCi*

If you want to play with your solution in GHCi, just run the command:

1 |
```
stack ghci
``` |

## Feedback, Issues, Pull Requests

The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

## Source

Sieve of Eratosthenes at Wikipedia http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.