It would be great to get a ForEach() formula.
Can you give an example of how you might use it 
We want to generate a url and need to add every relation to the url with a number. So every relation shoud get a number and other rules: app1=fibery&app2=make
Is it not possible to achieve this using .Join()
(maybe combined with some formula functions in the db containing the related items, if necessary)?
Yes, I managed everything apart from the numbering. So I would need the position.
How is the position defined? Is it a characteristic of the related items?
Or do you just want them numbered sequentially in the resultant URL?
Sequentially in the url would be great 
I was worried you might say that!
Yeah, there’s no easy way to create dynamic numbering within the join formula 
If the relationship is one-to-many, you could create a formula field in the ‘many’ database which will give you the sequence number of the entity, amongst all those that are linked to the same ‘one’:
One.Many.Filter([Creation Date] < [This Many].[Creation Date]).Count() + 1
Then you can use this in the .Join() function, something like this perhaps…
Many.Join("app" + SequenceNumber + "=" + Name,"&")