A small idea
It might be interesting to look at this as an extension of existing
Kernel.destructure/2:
https://hexdocs.pm/elixir/1.12/Kernel.html#destructure/2
Right now it works like this
destructure([x, y, z], [1, 2, 3, 4, 5]) # in other words,
destructure(form_with_bindings, data)
But if we provide another function, say destructure(data, form, new_form,
default \\ nil), with `new_form` specifying which data it should return
based on binds from `form`, we can easily use it in pipes like this:
data
|> ...
|> destructure({:ok, result}, result)
|> ...
|> destructure([a, b, c], %{a => 42 + b * c})
|> ...
On Wednesday, 14 December 2022 at 2:36:48 am UTC+1
matt.fa...@covermymeds.com wrote:
> Thank you for the useful feedback! Let me try addressing your excellent
> points in turn:
>
>
>
> 1. While `then/2` and `case/2` are somewhat suitable, they are
> verbose, especially in the case of the non-raising alternative. The
> additional typing involved for the anonymous function distracts the reader
> from the pattern matching, which is the crux of the matter.
> 2. With my implementation, the macro returns an empty string: assert
> “” == “$” |> pattern_filter(“$” <> money)
> 3. The `with/1` pipeline is verbose and lacks the visual flow of |>
> 4. The way I wrote the macro stipulates that there is only one
> non-underscored variable per pattern. Changing the examples you provided
> to
> `{a, _*b}` and `{*_a, b, _a}` would work just as well and remove
> ambiguity as to the return
> 1. So {1,2,1} |> pattern_filter({_a, b, _a}) is more concise and
> just as clear, imo
> 2. This does raise a warning, though I’m not sure why the warning
> is issued for a pattern like this. It just kind of (politely) tells me
> to
> remove the underscores, though the code works as intended.
>
>
>
>
>
>
>
>
>
>
>
> *From: *<elixir-l...@googlegroups.com> on behalf of Sabiwara Yukichi <
> sabi...@gmail.com>
> *Reply-To: *"elixir-l...@googlegroups.com" <elixir-l...@googlegroups.com>
> *Date: *Tuesday, December 13, 2022 at 6:55 PM
> *To: *"elixir-l...@googlegroups.com" <elixir-l...@googlegroups.com>
> *Subject: *[EXTERNAL] Re: [elixir-core:11213] [proposal] Use patterns to
> filter data (good for pipes)
>
>
>
> This email came from a source outside of CoverMyMeds. Use caution when
> clicking on links or replying with confidential information.
> ------------------------------
>
> This is an interesting idea, but maybe `then/2` (or case/2 if you're fine
> piping in it) could already cover these cases quite well (equivalent to
> your pattern_filter! function):
>
>
>
> "$3.45" |> then(fn "$" <> money -> money end) |> String.to_float()
>
> "$3.45" |> case do "$" <> money -> money end |> String.to_float()
>
>
>
> The non-raising alternative might be slightly more verbose because you
> need a second clause. But most of the time (like your money example), the
> next step of the pipe might fail on nil anyway. Or maybe a with/1 pipeline
> might work better than the pipe operator if you want a succession of
> happy-path matches?
>
>
>
> If we move forward, it might be better to explicitly declare the returned
> expression, not just infer it from dangling variables: it isn't obvious
> what should `{a, b}` or `{a, b, a}` return. Something like:
>
>
>
> {1, 2, 1} |> pattern_filter(b <- {a, b, a})
>
>
>
> Le mer. 14 déc. 2022 à 05:50, 'Matt Farabaugh' via elixir-lang-core <
> elixir-l...@googlegroups.com> a écrit :
>
> All,
>
>
>
> I wrote a macro which allows for using a pattern to essentially extract a
> value from a data structure:
>
>
>
>
> @doc """
> Filter a value out of data using a pattern with one variable. Returns a
> default value
> if no match. Raises if pattern contains no variables or more than one
> variable.
>
> ## Examples
>
> iex> "$3.45" |> PatternHelpers.pattern_filter("$" <> money) |>
> String.to_float()
> 3.45
>
> iex> {1,2,3} |> PatternHelpers.pattern_filter({1,_,a})
> 3
>
> iex> %{a: 1, b: 2} |> PatternHelpers.pattern_filter(%{a: 9, b: b})
> nil
>
> iex> %{a: 1, b: 2} |> PatternHelpers.pattern_filter(%{a: 9, b: b}, "???")
> "???"
> """
>
>
>
> And an unsafe version:
>
>
>
> @doc """
> See `pattern_filter/3`. Raises if no match.
>
> ## Examples
>
> iex> {1,2,3} |> PatternHelpers.pattern_filter!({9,_,b})
> ** (MatchError) no match of right hand side value: {1, 2, 3}
> """
>
>
>
> This is my first proposal. Please let me know if this idea is worth some
> attention, and how I might better do my part to present it. I have code
> obviously but I'm not sure this is the place for it.
>
>
>
> Thanks,
>
> Matt F
>
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