[Haskell-beginners] List Comprehensions

[Benjamin Edwards]

It needs to read x <- xs

You are wrapping a list in a list constructor :-)

On 11 May 2011 12:37, "Hamster" <machamster at gmail.com> wrote:

Hello,

I am trying to understand list comprehensions (I am very new to
Haskell) and there's one thing I simply don't understand.

If I have code that looks like:

combinations = [ (x,y) | x <-[1,2,3], y<-[1,2,3]]

Then it happily gives me all the possible combinations of x and y.
[(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)]

However...

If I change the function to look like:

combinations xs ys = [ (x,y) | x <-[xs], y<-[ys]]

The output changes to:
[([1,2,3],[1,2,3])]

I don't understand what's going on there. Is there any way I can get
it to produce all the combinations of a user supplied list?

Thanks

H.

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