# How to Test For Efficiency in Haskell

While Haskell isn't famous for it's efficiency, sometimes we still want to measure how long the evaluation of an expression takes.

To do so, you can use the `:set +s`

command in GHCi.

`ghci> :set +s`

You won't see any success message. Now you can test it:

```
ghci> let filterPrime (p:xs) = p : filterPrime [x | x <- xs, x `mod` p /= 0]
ghci> primes = filterPrime [2..]
ghci> :set +s
ghci> take 10 primes
[2,3,5,7,11,13,17,19,23,29]
(0.01 secs, 514,216 bytes)
```

When you're tracking the time of fast evaluations like the one above, you should evaluate the expression a couple of times and take the mean of all time values. Sometimes GHCi needs a bit time to power up, that's why an evalutation could take longer the first time.

This method works also with functions you've defined in an own .hs-file. You can remove the time evaluation by using `:unset +s`

.

Tip: If you're interested in prime number algorithms, check out the Sieve of Eratosthenes.