[Haskell-beginners] boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]
Daniel Fischer
daniel.is.fischer at googlemail.com
Tue Jun 21 22:24:55 CEST 2011
On Tuesday 21 June 2011, 21:58:05, anthony niro wrote:
> Hello,
>
> My name is Anthony i have few question about Haskell.
>
> 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
reads as
(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
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