Forgive me because I’m not 100% sure what you mean, but you can do this:
def update_nil_values(map, new_values, into \\ %{}) do
Enum.reduce(map, into, fn
{key, nil}, acc -> Map.put(acc, key, Map.get(new_values, key))
{key, value}, acc -> Map.put(acc, key, value)
end)
end
%MyStruct{a: nil, b: 10, c: nil}
|> update_nil_values(%{a: 20}, %MyStruct{})
|> other_stuff()
|> update_nil_values(%{c: 50}, %MyStruct{})
On Friday, 5 June 2020 15:26:36 UTC+1, alco wrote:
>
> Enum.reduce() does not compose as well when you have conditional Map.put
> spread over multiple private functions.
>
> On Fri, Jun 5, 2020 at 5:20 PM Adam Lancaster <
> [email protected] <javascript:>> wrote:
>
>> Thinking about it, can you solve the original problem with reduce?
>>
>> ```elixir
>> new_values = %{
>> foo: 10,
>> bar: 20
>> }
>>
>> Enum.reduce(map, %{}, fn
>> {key, nil}, acc -> Map.put(acc, key, Map.get(new_values, key))
>> {key, value}, acc -> Map.put(acc, key, value)
>> end)
>> ```
>>
>> Best
>>
>> Adam
>>
>> On 5 Jun 2020, at 15:10, Leandro Cesquini Pereira <[email protected]
>> <javascript:>> wrote:
>>
>> `maybe` is a common prefix for functions that may or may not do
>> something, eg:
>>
>>
>> https://github.com/elixir-lang/elixir/blob/779ccdb7564d587dfdadc1285a4dc57eb24768a1/lib/logger/lib/logger.ex#L1074
>>
>>
>> https://github.com/elixir-lang/elixir/blob/779ccdb7564d587dfdadc1285a4dc57eb24768a1/lib/elixir/lib/file.ex#L1796
>>
>> And so on...
>>
>> So I'd propose to call it `maybe_run`, just `maybe`, or something with
>> `maybe`.
>>
>> On Friday, June 5, 2020 at 9:48:56 AM UTC-4, Adam Lancaster wrote:
>>>
>>> We have something similar in our code base we called the Maybe.Pipe: ~>
>>>
>>>
>>> On 5 Jun 2020, at 14:41, Piotr Szmielew <[email protected]> wrote:
>>>
>>> maybe it should be called run_if? so, something like this:
>>>
>>> defmacro run_if(piped_value, condition, fun) do
>>> quote do
>>> if unquote(condition) do
>>> unquote(fun).(unquote(piped_value))
>>> else
>>> unquote(piped_value)
>>> end
>>> end
>>> end
>>>
>>> and then used as:
>>>
>>> %{a: 1, b: 2}
>>> |> X.run_if(1 == 2, &Map.put(&1, :foo, "foo"))
>>> |> X.run_if(1 == 1, &Map.put(&1, :foo, "bar"))
>>> # => %{a: 1, b: 2, foo: "bar"}
>>>
>>> On Fri, Jun 5, 2020 at 3:27 PM José Valim <[email protected]> wrote:
>>>
>>>> Hi Riccardo,
>>>>
>>>> This is an interesting proposal! Unfortunately if the value you want to
>>>> compute is expensive, then you need to fallback to the usual `if`
>>>> approach.
>>>> This is also specific to maps. It may be worth considering a more general
>>>> purpose mechanism, for example:
>>>>
>>>> map
>>>> |> update_if(foo != nil, &Map.put(&1, :foo, foo))
>>>> |> update_if(bar != nil, &Map.put(&1, :bar, normalize_bar(bar)))
>>>>
>>>> Being in Kernel also allows us to write it as a macro, which can be
>>>> further optimized. Not sure if update_if is the best name though.
>>>> Suggestions and comparisons to other languages are welcome.
>>>>
>>>> On Fri, Jun 5, 2020 at 12:36 PM Riccardo Binetti <[email protected]> wrote:
>>>>
>>>>> Hi everybody,
>>>>>
>>>>> this is my first proposal so I hope I get the format right, I've
>>>>> already checked in the mailing list and I haven't found similar proposals.
>>>>>
>>>>> *Problem*
>>>>>
>>>>> When manually constructing a map using external options or the result
>>>>> of a pattern match on a struct, I often find myself in the situation
>>>>> where
>>>>> I have to add a value only if it is non-nil.
>>>>>
>>>>> The possible approaches available today are these
>>>>>
>>>>>
>>>>> *Using if*
>>>>>
>>>>>
>>>>> def some_fun(s) do
>>>>> MyStruct{
>>>>> foo: foo,
>>>>> bar: bar,
>>>>> baz: baz
>>>>> } = s
>>>>>
>>>>> map = %{a: "hello", b: 42}
>>>>>
>>>>> map =
>>>>> if foo do
>>>>> Map.put(map, :foo, foo)
>>>>> else
>>>>> map
>>>>> end
>>>>>
>>>>> map =
>>>>> if bar do
>>>>> Map.put(map, :bar, bar)
>>>>> else
>>>>> map
>>>>> end
>>>>>
>>>>> if bar do
>>>>> Map.put(map, :baz, baz)
>>>>> else
>>>>> map
>>>>> end
>>>>> end
>>>>>
>>>>>
>>>>> This is the obvious solution, but it gets very verbose very quickly
>>>>>
>>>>>
>>>>> *Populating the map and then filtering*
>>>>>
>>>>> def some_fun(s) do
>>>>> MyStruct{
>>>>> foo: foo,
>>>>> bar: bar,
>>>>> baz: baz
>>>>> } = s
>>>>>
>>>>> %{a: "hello", b: 42}
>>>>> |> Map.put(:foo, foo)
>>>>> |> Map.put(:bar, bar)
>>>>> |> Map.put(:baz, baz)
>>>>> |> Enum.filter(fn {_k, v} -> v end)
>>>>> |> Enum.into(%{})
>>>>> end
>>>>>
>>>>> This is more concise, but it creates intermediate maps that are
>>>>> immediately thrown away. Moreover, if the initial map is very large, the
>>>>> filter operation is expensive since it traverses the whole map.
>>>>>
>>>>>
>>>>> *Implementing a maybe_put helper function*
>>>>>
>>>>> def some_fun(s) do
>>>>> MyStruct{
>>>>> foo: foo,
>>>>> bar: bar,
>>>>> baz: baz
>>>>> } = s
>>>>>
>>>>> %{a: "hello", b: 42}
>>>>> |> maybe_put(:foo, foo)
>>>>> |> maybe_put(:bar, bar)
>>>>> |> maybe_put(:baz, baz)
>>>>> end
>>>>>
>>>>> defp maybe_put(map, _key, nil), do: map
>>>>> defp maybe_put(map, key, value), do: Map.put(map, key, value)
>>>>>
>>>>> This is the solution I end implementing almost always. It is present
>>>>> in multiple modules across the codebase I work on (and/or put in a
>>>>> generic
>>>>> Utils module), and talking with my colleagues I found out that is a
>>>>> utility
>>>>> function that they also have to implement often and it has turned up at
>>>>> least a couple times[1][2] also in the Elixir Forum.
>>>>>
>>>>> [1]
>>>>> https://elixirforum.com/t/elixir-way-to-conditionally-update-a-map/17952/21?u=rbino
>>>>> [2]
>>>>> https://elixirforum.com/t/is-there-way-to-create-map-and-conditionally-exclude-some-keys/23315/5?u=rbino
>>>>>
>>>>> This brings me to my proposal, which is a more generic version of
>>>>> maybe_put.
>>>>>
>>>>> *Proposal*
>>>>>
>>>>> Add Map.put_if/4 to the Map module. This is more generic (and imho
>>>>> clearer) than maybe_put and can be used to solve the same problem without
>>>>> creating intermediate results and allowing the code to be clear and
>>>>> concise.
>>>>>
>>>>>
>>>>> def some_fun(s) do
>>>>> MyStruct{
>>>>> foo: foo,
>>>>> bar: bar,
>>>>> baz: baz
>>>>> } = s
>>>>>
>>>>> %{a: "hello", b: 42}
>>>>> |> Map.put_if(:foo, foo, foo != nil)
>>>>> |> Map.put_if(:bar, bar, bar != nil)
>>>>> |> Map.put_if(:baz, baz, baz != nil)
>>>>> end
>>>>>
>>>>> Let me know if you need any other information.
>>>>> --
>>>>> Riccardo
>>>>>
>>>>> --
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>>>>> an email to [email protected].
>>>>> To view this discussion on the web visit
>>>>> https://groups.google.com/d/msgid/elixir-lang-core/f513b901-fd93-4f28-88a2-f496a3c84a21o%40googlegroups.com
>>>>>
>>>>> <https://groups.google.com/d/msgid/elixir-lang-core/f513b901-fd93-4f28-88a2-f496a3c84a21o%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>>> .
>>>>>
>>>>
>>>> --
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>>>> To view this discussion on the web visit
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>>>>
>>>> <https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4K_QVpD-eRG6JUOMYV5pVTLchpTgbO0fqN6pPCmuSL4NA%40mail.gmail.com?utm_medium=email&utm_source=footer>
>>>> .
>>>>
>>>
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>>> .
>>>
>>>
>>>
>> --
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>>
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>>
>
>
> --
> Best regards
> Alexei Sholik
>
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