# [Haskell-beginners] boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]

Tue Jun 21 22:24:55 CEST 2011

```On Tuesday 21 June 2011, 21:58:05, anthony niro wrote:
> Hello,
>
>
> I was reading a tutorial everything goes well. but now i have few things
> that i dont understang.
>
> HERE:
>
> boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]

That's a "list comprehension", if you want further information, searching
for that term should turn up a few sources.

>
> I dont understand this line except the "if" "then" "else".
> What is xs?

xs is the argument of the function boomBangs, it is a list.
(mnemonic: xs is the plural of x, it's common to denote single elements of
a list by x, y, k, n, ... and the corresponding lists of [potentially]
several such elements xs, ys, ...)

> what is the |  ?

It's a separator. It separates the elements of the result list from the
generators and predicates.
List comprehension syntax is similar to set comprehension syntax, the above

(if x < 10 then "BOOM!" else "BANG!") where x runs through/is drawn from xs
and x is odd.

>  and why doing this  "  | x <- xs, odd x]"
>
> why x <- xs????? what is that

"x <- xs" is the 'generator', you could also read that as "for all x in xs"
(but, as you will soon discover, it's more general, that works also for
other things than lists).

>
> and what is odd x?

That's a predicate or test, `odd' is a function which tests whether a
number is odd, hence `odd x' tells you whether x is an odd number.
Appearing in that position, it acts as a filter, the expression to the left
of the separator (`|', to reiterate) is only included in the result list if
the test evaluates to True.

HTH,
Daniel

```