Hi Burdock,
for-comprehensions map conceptually to flat_map (implementation-wise they
use reduce though). So the code above could be written as:
Enum.flat_map([:a,:b,:c], fn i ->
Enum.flat_map([1, 2], fn j ->
{i, j}
end)
end)
The benefit is that it just works with Stream.
*José Valim*
www.plataformatec.com.br
Skype: jv.ptec
Founder and Director of R&D
On Fri, Jul 28, 2017 at 2:19 AM, Burdock <[email protected]> wrote:
> Hello everyone,
>
> One of the most attractive features of list comprehensions is their
> ability to avoid nested loops
> for i <- [:a, :b, :c], j <- [1, 2], do: {i, j} # Happy dev :)
>
> flat_map([:a, :b, :c],
> fn i ->
> map([1,2],
> fn j ->
> {i, j}
> end)
> end) # Sad dev :(
>
> Something that I have wanted for a long time is a way to do the same thing
> as list comprehensions, but with the Enum syntax.
> Enum.product([ [:a,:b,:c], [1,2] ])
> |> Enum.map( {i, j} -> ... end) # Same tuple syntax as zip
>
> This *could* be extended further by adding nested access like *get_in/2*
>
> carts = [
> products: [
> %{name: "shirt",
> coupons: 101, 202
> },%{name: "pants",
> coupons: 303, 404
> }]
> }, %{
> products: [
> %{name: "T-shirt",
> coupons: 101, 202
> },%{name: "shorts",
> coupons: 303, 404
> }]
> }
> ]
>
> Enum.product(carts, [:products, :coupons])
> |> Enum.map( {cart, product, coupon} -> ... end) # Every Layer of
> nesteing is kept available via tuple syntax
>
> # Alternative syntax where Enum.product/2 :: left, right ->
> Enum.product([left, right]) would work
>
> Enum.product(carts, :products)
> |> Enum.product(:coupons)
> |> Enum.map( {cart, product, coupon} -> ... end) # Every Layer of
> nesteing is kept available via tuple syntax
>
> This is cool and all, but what about Stream.product? Unfortunately,
> Streams are not quite as simple. Streams don't always have a finite length.
> natrual_numbers = Stream.unfold(1, fn n -> n + 1 end)
> for i <- natrual_numbers, j <- natrual_numbers, do: {i, j} # ????????
>
> I don't want to get into too much depth on Stream.product here because
> making it work safely with Streams of unknown length requires a somewhat
> math heavy explanation. If we get the green light on Enum.product I will
> make the suggestion for Stream.product. The only thing that's important
> is that it is possible via some higher dimensional pairing function magic :)
>
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