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
* So {1,2,1} |> pattern_filter({_a, b, _a}) is more concise and just as
clear, imo
* 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-lang-core@googlegroups.com> on behalf of Sabiwara Yukichi
<sabiw...@gmail.com>
Reply-To: "elixir-lang-core@googlegroups.com"
<elixir-lang-core@googlegroups.com>
Date: Tuesday, December 13, 2022 at 6:55 PM
To: "elixir-lang-core@googlegroups.com" <elixir-lang-core@googlegroups.com>
Subject: [EXTERNAL] Re: [elixir-core:11213] [proposal] Use patterns to filter
data (good for pipes)
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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-lang-core@googlegroups.com<mailto:elixir-lang-core@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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