### Infinite Structures

blogimage("escher_infinite_lizards.jpg","Escher")
It is often said that Haskell is _lazy_.
In fact, if you are a bit pedantic, you should say that [Haskell is _non-strict_](http://www.haskell.org/haskellwiki/Lazy_vs._non-strict).
Laziness is just a common implementation for non-strict languages.
Then what does "not-strict" mean? From the Haskell wiki:
> Reduction (the mathematical term for evaluation) proceeds from the outside in.
>
> so if you have `(a+(b*c))` then you first reduce `+` first, then you reduce the inner `(b*c)`
For example in Haskell you can do:
> -- numbers = [1,2,..]
> numbers :: [Integer]
> numbers = 0:map (1+) numbers
>
> take' n [] = []
> take' 0 l = []
> take' n (x:xs) = x:take' (n-1) xs
>
> main = print $ take' 10 numbers
And it stops.
How?
Instead of trying to evaluate `numbers` entirely,
it evaluates elements only when needed.
Also, note in Haskell there is a notation for infinite lists
~~~
[1..] ⇔ [1,2,3,4...]
[1,3..] ⇔ [1,3,5,7,9,11...]
~~~
and most functions will work with them.
Also, there is a built-in function `take` which is equivalent to our `take'`.