[Haskell-beginners] another list comprehesion error

[Thomas]

Hi!

On 21.07.2011 21:29, Roelof Wobben wrote:
> roelof n x = [x | y<- [1..n]]

Have you tried this?
What do you get for, say "roelof 4 5" ? Why?

Now try this instead:
   roelof' n = [x | x <- [1..n]]
What do you get for "roelof' 4" ?

> So I thought the syntax of a list compression would be [output | filter<- input]
> But the answer is now  [ output | input<- filter] This is very confusing for me.

Hm, in the terminology of the book you're using the list comprehension
    [x | x <- [1..n]] reads
"all x such that x drawn from [1..n]"

So, you have a result (x) and  a generator (x <- [1..n]).
What you call "filter" is called "guard" in the book.
So actually you have:
   [ result | generator(s) ]
or (with guards):
   [ result | generator(s), guards(s) ]

So, note that the comma (,) is a mere separator and the left arrow (<-) 
forms part of the generators (everything else is a guard or an error). 
Especially the term "input" is rather misleading.

Generators are explained in chapter 5.1 (page 38f), guards are explained 
in 5.2 (page 39f). I'm not sure I can explain better than the book. 
Maybe you should experiment with generators first and then advance to 
the guards.

A few suggestions (without guards):
The list of the first 5 natural numbers.
The list of the first 5 odd numbers.
The list of all pairs (a, b) where a,b > 0, a <= b and b <= 5

Now you can try the last two with guards.

HTH,
Thomas