I can't get Enum.into/2 to work according to spec

I want to preface this by saying that I’m feeling very thankful for all the helpful answers I’ve been getting from the community and feel like I’m to the point where going to the source code first is really productive.

Getting into the difference between Enumerable and Collectable protocols and how they are invoked in the Enum and Stream modules, I ran across something that didn’t make sense to me.

Setting aside the deprecated collectables (non-empty and keyword lists), I understand the following collections to implement the two different protocols:

Enumerable Collectable (supported)
IO.Stream IO.Stream
Range Empty List
List BitString
Map Map
Function MapSet
MapSet Mix.Shell
DateRange File.Stream
Stream Stream

I haven’t been able to reconcile the spec typing with what seems to work.

@spec into(Enumerable.t(), Collectable.t()) :: Collectable.t()
Enum.into(enumerable, collectable)

Some situations probably aren’t that helpful like passing a range into an empty map but should it work if they typing says they should?

Enum.into(1..10//2, %{})
** (FunctionClauseError) no function clause matching in anonymous fn/2 in Collectable.Map.into/1    
    The following arguments were given to anonymous fn/2 in Collectable.Map.into/1:
        # 1
        # 2
        {:cont, 1}
    (elixir 1.15.7) anonymous fn/2 in Collectable.Map.into/1
    (elixir 1.15.7) lib/enum.ex:1554: anonymous fn/3 in Enum.reduce_into_protocol/3
    (elixir 1.15.7) lib/range.ex:526: Enumerable.Range.reduce/5
    (elixir 1.15.7) lib/enum.ex:1553: Enum.reduce_into_protocol/3
    (elixir 1.15.7) lib/enum.ex:1537: Enum.into_protocol/2
    /Users/bradhanks/membrane_tutorial.livemd#cell:wp5kyzkt6fqb75ktz3vdbl7sbfehlmzc:1: (file)

It makes sense that the Collectable.Map.into/2 expects a tuple and that Collectable.BitString.into/2 only has function clauses that match for binary or bitstring.

I’m looking for a better understanding on what the types in @spec represent.

I don’t think you can collect a range in a map because a map needs a key and a value.

For instance 1..10//2 |> Enum.map(& {&1, :some_value}) |> Enum.into(%{}) should work.

Edit: but you know that, sorry I did not read your post carefully. Well the types are broad and do not have generics.

I a static language you would have Enumerable.t<Pair> and Collectable.t<Pair> and it would not compile unless the wrapped types are compatible (Pair and Pair are compatible for instance).


Some typings, particularly for generic functions like in Enum, are intended more for human readers than for the compiler.

For instance, both Enumerable.t() and Collectable.t() are generated by Protocol as aliases of term:

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The same is true the other way as well. Maps are an enumerable of tuple-2 values. The same the map collectable implementation expects an enumerable of tuple-2 values as input.


Yeah that was going to be my next question. I held off just in case the answer to this resolved the other. Thank you for clearing that up!

Arguably there isn’t much utility to the change but it would be tidier if there was some sort of provision for defaults when required.

# map_acc, {:cont, {:default_key, default_key, value }} -> Map.put(map_cc, default_key, value)
map_acc, {:cont, {key, value}} -> Map.put(map_cc, key value)
map_acc, :done -> map_acc
_map_acc, :halt -> :ok:

Yeah there seem to be many instances when it isn’t going to work which is why it was so disconcerting. But I’m not that bothered if no one else is. There’s always into/3.

Hi @BradS2S! You have been submitting PRs to Elixir and if you want to send a PR that improves the error message here, it would be very welcome. Something like:

map_acc, {:cont, {key, value}} -> Map.put(map_cc, key value)
map_acc, {:cont, other} -> raise "collecting into a map requires {key, value} tuples, got: #{inspect(other)}"

Thank you!


Thank you! I’ll do it. :grinning: