How do I return/yield a value from a recursive function in Elixir?

Question

this seems like a simple thing to do but I just don't seem to find a simple enough answer anywhere for me to grok.

So lets say I have an Elixir function like this:

   def fetch_card_ids(offset, total_cards) when offset < total_cards do
       url = "http://some.domanin?offset=#{offset}"
       response = HTTPotion.get url

       #return or yield response here

       fetch_card_ids(offset+24,total_cards)
   end

In C# I could yield a return value when iterating something which of course is not the same as recursion but nevertheless: can I do a similar thing in a recursive function in elixir?

Any help would be much appreciated of course.

whatyouhide · Accepted Answer · 2015-12-29 08:24:38Z

5

If what you want to do is "stream" the response to the caller, then Streams are a good way to go. For example, you could use Stream.unfold/2:

def fetch_card_ids(offset, total_cards) do
  Stream.unfold offset, fn
    off when off < total_cards ->
      resp = HTTPotion.get(url)
      {resp, off + 24}
    _ ->
      nil
  end
end

This stream will "yield" responses every time it fetches them and stop when offset gets greater than total_cards.

edited Dec 29, 2015 at 8:24

answered Dec 27, 2015 at 9:59

whatyouhide

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2 Comments

kaqqao Over a year ago

I'd say you typo'd at Stream.iterate there. You meant Stream.unfold, no?

whatyouhide Over a year ago

@kaqqao whoops, nice catch! Fixed :)

GavinBrelstaff · Accepted Answer · 2015-12-27 20:06:15Z

2

How about using Streams this way

top = div(total_cards,24) +1 # integer division
result = Stream.map( 0..top, fn (offset) -> 
  url = "http://some.domanin?offset=#{24*offset}"
  HTTPotion.get url
  end ) |> Enum.to_list

Here Enum.to_list simply "yields" the list at the end but you can also process from that list in progressive time using things like Stream.take etc

edited Dec 27, 2015 at 20:06

answered Dec 27, 2015 at 11:24

GavinBrelstaff

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