> Alternatively, the `$` symbol could be used at the beginning of the data
structure to indicate that it is performing capture destructuring (e.g.,
`$%{key1:, key2:}` or `$%{"key1", "key2"}`, but then it starts feeling a
little more line-noisy.
I agree that'd be noisy. Also, it might make mixing tagged variable
literals, literal => pairs, and trailing keyword pairs even more confusing.
Consider today that we support:
%{"fizz" => "buzz", foo: :bar}
# => %{:foo => :bar, "fizz" => "buzz"}
But do not support:
%{foo: :bar, "fizz" => "buzz"}
# !> ** (SyntaxError) invalid syntax found on iex:5:12:
# !> ┌─ error: iex:5:12
# !> │
# !> 5 │ %{foo: :bar, "fizz" => "buzz"}
# !> │ ^
# !> │
# !> unexpected expression after keyword list. Keyword lists must
always come last in lists and maps. Therefore, this is not allowed:
# !>
# !> [some: :value, :another]
# !> %{some: :value, another => value}
# !>
# !> Instead, reorder it to be the last entry:
# !>
# !> [:another, some: :value]
# !> %{another => value, some: :value}
# !>
# !> Syntax error after: ','
Supporting $%{key1:, key2:} or $%{"key1", "key2"} obfuscates this situation
even further.
On Wednesday, June 28, 2023 at 10:16:10 PM UTC-5 halos...@gmail.com wrote:
> On Wed, Jun 28, 2023 at 8:41 PM Paul Schoenfelder <
> paulscho...@fastmail.com> wrote:
>
>> I have an almost visceral reaction to the use of capture syntax for this
>> though, and I don’t believe any of the languages you mentioned that support
>> field punning do so in this fashion. They all use a similar intuitive
>> syntax where the variable matches the field name, and they don’t make any
>> effort to support string keys.
>>
>
> JavaScript *only* supports string keys. Ruby’s pattern matching which can
> lead to field punning only supports symbol keys, but since ~2.2 Ruby can
> garbage collect symbols, making it *somewhat* less dangerous to do
> `JSON.parse!(data, keys: :symbol)` than it was previously.
>
> As far as I know, the BEAM does not do any atom garbage collection, and
> supporting *only* symbols will lead to a greater chance of atom exhaustion
> because a non-flagged mechanism here that only works on atom keys will lead
> to `Jason.parse(data, keys: :atom)` (and not `Jason.parse(data, keys:
> :atom!)`). I do not think that any destructuring syntax which works on maps
> with symbol keys but not string keys will be acceptable, although if it is
> constrained to *only* work on structs, then it does not matter (as that is
> the same restriction that it appears that OCaml and Haskell have).
>
> I think that either `&:key` / `&"key"` or `$:key` / `$"key"` will work
> very nicely for this feature, although it would be nice to have `&key:` or
> `$key:` work the same as the former version. Alternatively, the `$` symbol
> could be used at the beginning of the data structure to indicate that it is
> performing capture destructuring (e.g., `$%{key1:, key2:}` or `$%{"key1",
> "key2"}`, but then it starts feeling a little more line-noisy.
>
> I think that the proposal here — either using `&` or `$` — is entirely
> workable and IMO extends the concept nicely.
>
> -a
>
> On Wed, Jun 28, 2023, at 7:56 PM, Christopher Keele wrote:
>>
>> This is a formalization of my concept here
>> <https://groups.google.com/g/elixir-lang-core/c/oFbaOT7rTeU/m/BWF24zoAAgAJ>,
>> as a first-class proposal for explicit discussion/feedback, since I now
>> have a working prototype
>> <https://github.com/elixir-lang/elixir/compare/main...christhekeele:elixir:tagged-variable-capture>
>> .
>>
>> *Goal*
>>
>> The aim of this proposal is to support a commonly-requested feature:
>> *short-hand
>> construction and pattern matching of key/value pairs of associative data
>> structures, based on variable names* in the current scope.
>>
>> *Context*
>>
>> Similar shorthand syntax sugar exists in many programming languages
>> today, known variously as:
>>
>> - Field Punning <https://dev.realworldocaml.org/records.html> — OCaml
>> - Record Puns
>>
>> <https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/record_puns.html>
>> — Haskell
>> - Object Property Value Shorthand
>>
>> <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#property_definitions>
>>
>> — ES6 Javascript
>>
>> This feature has been in discussion for a decade, on this mailing list (1
>> <https://groups.google.com/g/elixir-lang-core/c/4w9eOeLvt-8/m/WOkoPSMm6kEJ>,
>> 2
>> <https://groups.google.com/g/elixir-lang-core/c/NoUo2gqQR3I/m/WTpArTGMKSIJ>,
>> 3
>> <https://groups.google.com/g/elixir-lang-core/c/3XrVXEVSixc/m/NHU2M4QFAQAJ>,
>> 4
>> <https://groups.google.com/g/elixir-lang-core/c/OvSQkvXxsmk/m/bKKHbBxiCwAJ>,
>> 5
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/1W-d_XAlBgAJ>
>> , 6 <https://groups.google.com/g/elixir-lang-core/c/oFbaOT7rTeU>)
>> and the Elixir forum (1
>> <https://elixirforum.com/t/proposal-add-field-puns-map-shorthand-to-elixir/15452>,
>>
>> 2
>> <https://elixirforum.com/t/shorthand-for-passing-variables-by-name/30583>,
>> 3
>> <https://elixirforum.com/t/if-you-could-change-one-thing-in-elixir-language-what-you-would-change/19902/17>,
>>
>> 4
>> <https://elixirforum.com/t/has-map-shorthand-syntax-in-other-languages-caused-you-any-problems/15403>,
>>
>> 5
>> <https://elixirforum.com/t/es6-ish-property-value-shorthands-for-maps/1524>,
>> 6
>> <https://elixirforum.com/t/struct-creation-pattern-matching-short-hand/7544>),
>>
>> and has motivated many libraries (1
>> <https://github.com/whatyouhide/short_maps>, 2
>> <https://github.com/meyercm/shorter_maps>, 3
>> <https://hex.pm/packages/shorthand>, 4 <https://hex.pm/packages/synex>).
>> These narrow margins cannot fit the full history of possibilities,
>> proposals, and problems with this feature, and I will not attempt to
>> summarize them all. For context, I suggest reading this mailing list
>> proposal
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/1W-d_XAlBgAJ>
>> and this community discussion
>> <https://elixirforum.com/t/proposal-add-field-puns-map-shorthand-to-elixir/15452>
>> in
>> particular.
>>
>> However, in summary, this particular proposal tries to solve a couple of
>> past sticking points:
>>
>> 1. Atom vs String
>>
>> <https://groups.google.com/g/elixir-lang-core/c/NoUo2gqQR3I/m/IpZQHbZk4xEJ>
>> key support
>> 2. Visual clarity
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/NBkAVto0BAAJ>
>> that atom/string matching is occurring
>> 3. Limitations of string-based sigil parsing
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/TiZw6xM3BAAJ>
>> 4. Easy confusion
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/WRhXxHDfBAAJ>
>> with tuples
>>
>> I have a working fork of Elixir here
>> <https://github.com/christhekeele/elixir/tree/tagged-variable-capture>
>> where this proposed syntax can be experimented with. Be warned, it is buggy.
>>
>> *Proposal: Tagged Variable Captures*
>>
>> I propose we overload the unary capture operator (*&*) to accept
>> compile-time atoms and strings as arguments, for example *&:foo* and
>> *&"bar"*. This would *expand at compile time* into *a tagged tuple with
>> the atom/string and a variable reference*. For now, I am calling this a
>> *"tagged-variable
>> capture"* to differentiate it from a function capture.
>>
>> For the purposes of this proposal, assume:
>>
>> {foo, bar} = {1, 2}
>>
>> Additionally,
>>
>> - Lines beginning with *# == * indicate what the compiler expands an
>> expression to.
>> - Lines beginning with *# => * represent the result of evaluating
>> that expression.
>> - Lines beginning with *# !> * represent an exception.
>>
>> *Bare Captures*
>>
>> I'm not sure if we should support *bare* tagged-variable capture, but it
>> is illustrative for this proposal, so I left it in my prototype. It would
>> look like:
>>
>> &:foo
>> *# == **{:foo, foo}*
>> *# => *{:foo, 1}
>> &"foo"
>> *# == **{"foo", foo}*
>> *# => *{"foo", 1}
>>
>> If bare usage is supported, this expansion would work as expected in
>> match and guard contexts as well, since it expands before variable
>> references are resolved:
>>
>> {:foo, baz} = &:foo
>> *# == {:foo, baz} = {:foo, foo}*
>> *# => *{:foo, 1}
>> baz
>> *# => *1
>>
>> *List Captures*
>>
>> Since capture expressions are allowed in lists, this can be used to
>> construct Keyword lists from the local variable scope elegantly:
>>
>> list = [&:foo, &:bar]
>> *# == **list = [{:foo, foo}, {:bar, bar}]*
>> *# => *[foo: 1, bar: 2]
>>
>> This would work with other list operators like *|*:
>>
>> baz = 3
>> list = [&:baz | list]
>> *# == **list = [**{:baz, baz} **| **list**]*
>> *# => *[baz: 3, foo: 1, bar: 2]
>>
>> And list destructuring:
>>
>> {foo, bar, baz} = {nil, nil, nil}
>> [&:baz, &:foo, &:bar] = list
>> *# == [{:baz, baz}, {:foo, foo}, {:bar, bar}] = list*
>> *# => *[baz: 3, foo: 1, bar: 2]
>> {foo, bar, baz}
>> *# => *{1, 2, 3}
>>
>> *Map Captures*
>>
>> With a small change to the parser,
>> <https://github.com/elixir-lang/elixir/commit/0a4f5376c0f9b4db7d71514d05df6b8b6abc96a9>
>>
>> we can allow this expression inside map literals. Because this expression
>> individually gets expanded into a tagged-tuple before the map associations
>> list as a whole are processed, it allow this syntax to work in all existing
>> map/struct constructs, like map construction:
>>
>> map = %{&:foo, &"bar"}
>> *# == %{:foo => foo, "bar" => bar}*
>> *# => *%{:foo => 1, "bar" => 2}
>>
>> Map updates:
>>
>> foo = 3
>> map = %{map | &:foo}
>> *# == %{map | :foo => foo}*
>> *# => *%{:foo => 3, "bar" => 2}
>>
>> And map destructuring:
>>
>> {foo, bar} = {nil, nil}
>> %{&:foo, &"bar"} = map
>> *# == %{:foo => foo, "bar" => bar} = map*
>> *# => *%{:foo => 3, "bar" => 2}
>> {foo, bar}
>> *# => *{3, 2}
>>
>> *Considerations*
>>
>> Though just based on an errant thought
>> <https://groups.google.com/g/elixir-lang-core/c/oFbaOT7rTeU/m/BWF24zoAAgAJ>
>> that popped into my head yesterday, I'm unreasonably pleased with how well
>> this works and reads in practice. I will present my thoughts here, though
>> again I encourage you to grab my branch
>> <https://github.com/christhekeele/elixir/tree/tagged-variable-capture>,
>> compile
>> it from source
>> <https://github.com/christhekeele/elixir/tree/tagged-variable-capture#compiling-from-source>,
>> and
>> play with it yourself!
>>
>> *Pro: solves existing pain points*
>>
>> As mentioned, this solves flaws previous proposals suffer from:
>>
>> 1. Atom vs String
>>
>> <https://groups.google.com/g/elixir-lang-core/c/NoUo2gqQR3I/m/IpZQHbZk4xEJ>
>> key
>> support
>> This supports both.
>> 2. Visual clarity
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/NBkAVto0BAAJ>
>> that
>> atom/string matching is occurring
>> This leverages the appropriate literal in question within the syntax
>> sugar.
>> 3. Limitations of string-based sigil parsing
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/TiZw6xM3BAAJ>
>> This is compiler-expansion-native.
>> 4. Easy confusion
>>
>> <https://groups.google.com/g/elixir-lang-core/c/XxnrGgZsyVc/m/WRhXxHDfBAAJ>
>> with
>> tuples
>> %{&:foo, &"bar"} is very different from {foo, bar}, instead of
>> 1-character different.
>>
>> Additionally, it solves my main complaint with historical proposals:
>> syntax to combine a variable identifier with a literal must either obscure
>> that we are building an identifier, or obscure the key/string typing of the
>> literal.
>>
>> I'm proposing overloading the capture operator rather than introducing a
>> new operator because the capture operator already has a semantic
>> association with messing with variable scope, via the nested integer-based
>> positional function argument syntax (ex *& &1*).
>>
>> By using the capture operator we indicate that we are messing with an
>> identifier in scope, but via a literal atom/string we want to associate
>> with, to get the best of both worlds.
>>
>> *Pro: works with existing code*
>>
>> The capture today operator has well-defined compile-time-error semantics
>> if you try to pass it an atom or a string. All compiling Elixir code today
>> will continue to compile as before.
>>
>> *Pro: works with existing tooling*
>>
>> By overloading an existing operator, this approach works seamlessly for
>> me with the syntax highlighters I have tried it with so far, and reasonable
>> with the formatter.
>>
>> In my experimentation I've found that the formatter wants to rewrite *&:baz
>> *to *(&:baz)* pretty often. That's good, because there are several edge
>> cases in my prototype where not doing so causes it to behave strangely; I'm
>> sure it's resolving ambiguities that would occur in function captures that
>> impact my proposal in ways I have yet fully anticipated.
>>
>> *Pros: minimizes surface area of the language*
>>
>> By overriding the capture operator instead of introducing a new operator
>> or sigil, we are able to keep the surface area of this feature slim.
>>
>> *Cons: overloads the capture operator*
>>
>> Of course, much of the virtues of this proposal comes from overloading
>> the capture operator. But it is an already semantically fraught syntactic
>> sugar construct that causes confusion to newcomers, and this would place
>> more strain on it.
>>
>> We would need to augment it with more than the meager error message
>> modification
>> <https://github.com/elixir-lang/elixir/commit/3d83d21ada860d03cece8c6f90dbcf7bf9e737ec#diff-92b98063d1e86837fae15261896c265ab502b8d556141aaf1c34e67a3ef3717cL199-R207>
>> in
>> my prototype, as well as documentation and anticipate a new wave of
>> questions from the community upon release.
>>
>> This inelegance really shows when considering embedding a tagged variable
>> capture inside an anonymous function capture, ex *& &1 = &:foo*. In my
>> prototype I've chosen to allow this rather than error on "nested captures
>> not allowed" (would probably become: "nested *function* captures not
>> allowed"), but I'm not sure I found all the edge-cases of mixing them in
>> all possible constructions.
>>
>> Additionally, since my proposal now allows the capture operator as an
>> associative element inside map literal parsing, that would change the
>> syntax error reported by providing a function capture as an associative
>> element to be generated during expansion rather than during parsing. I am
>> not fluent enough in leex to have have updated the parser to preserve the
>> exact old error, but serendipitously what it reports in my prototype today
>> is pretty good regardless, but I prefer the old behaviour:
>>
>> Old:
>> %{& &1}
>> *# !> **** (SyntaxError) syntax error before '}'*
>> *# !> * |
>> *# !> * 1 | %{& &1}
>> *# !> * | ^
>> New:
>> %{& &1}
>> *# => error: expected key-value pairs in a map, got: & &1*
>> *# => ** (CompileError) cannot compile code (errors have been logged)*
>>
>> *Cons: here there be dragons I cannot see*
>>
>> I'm quite sure a full implementation would require a lot more knowledge
>> of the compiler than I am able to provide. For example, *&:foo = &:foo
>> *raises
>> an exception where *(&:foo) = &:foo* behaves as expected. I also find
>> the variable/context/binding environment implementation in the erlang part
>> of the compiler during expansion to be impenetrable, and I'm sure my
>> prototype fails on edge cases there.
>>
>> *Open Question: the pin operator*
>>
>> As this feature constructs a variable ref for you, it is not clear if/how
>> we should support attempts to pin the generated variable to avoid new
>> bindings. In my prototype, I have tried to support the pin operator via the
>> *&^:atom *syntax, though I'm pretty sure it's super buggy on bare
>> out-of-data-structure cases and I only got it far enough to work in
>> function heads for basic function head map pattern matching.
>>
>> *Open Question: charlists*
>>
>> I did not add support for charlist tagged variable captures in my
>> prototype, as it would be more involved to differentiate a capture of list
>> mean to become a tagged tuple from a list representing the AST of a
>> function capture. I would not lose a lot of sleep over this.
>>
>> *Open Question: allowed contexts*
>>
>> Would we even want to allow this syntax construct outside of map
>> literals? Or list literals?
>>
>> I can certainly see people abusing the
>> bare-outside-of-associative-datastructure syntax to make some neigh
>> impenetrable code where it's really unclear where assignment and pattern
>> matching is occuring, and relatedly this is where I see a lot of odd
>> edge-case behaviour in my prototype. I allowed it to speed up the
>> implementation, but it merits more discussion.
>>
>> On the other hand, this does seem like an... interesting use-case:
>>
>> error = "rate limit exceeded"
>> &:error *# return error tuple*
>>
>> *Thanks for reading! What do you think?*
>>
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "elixir-lang-core" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to elixir-lang-co...@googlegroups.com.
>> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/elixir-lang-core/ad7e0313-4207-4cb7-a5f3-d824735830abn%40googlegroups.com
>>
>> <https://groups.google.com/d/msgid/elixir-lang-core/ad7e0313-4207-4cb7-a5f3-d824735830abn%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>>
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "elixir-lang-core" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to elixir-lang-co...@googlegroups.com.
>>
> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/elixir-lang-core/4ee25f02-f27e-47a8-b4b5-b8520c1c9b05%40app.fastmail.com
>>
>> <https://groups.google.com/d/msgid/elixir-lang-core/4ee25f02-f27e-47a8-b4b5-b8520c1c9b05%40app.fastmail.com?utm_medium=email&utm_source=footer>
>> .
>>
>
>
> --
> Austin Ziegler • halos...@gmail.com • aus...@halostatue.ca
> http://www.halostatue.ca/ • http://twitter.com/halostatue
>
--
You received this message because you are subscribed to the Google Groups
"elixir-lang-core" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to elixir-lang-core+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/elixir-lang-core/d1af9191-735c-4042-8c01-2c8ec0c73bc6n%40googlegroups.com.
