%{$:foo, "fizz" => "buzz", $"bar", fizz: :buzz, $:baz}
Personally, my preference would be to disallow this usage, but perhaps with
an even more instructive compiler error message.
In fact, I think that we could leverage most existing errors/warnings
today, as long as things like the compiler error reporter desugar this
feature before reporting, to make it clearer upon error what is actually
going on in a variety of circumstances. This would give us something more
like:
%{$:foo, "fizz" => "buzz", $"bar", fizz: :buzz, $:baz} # !> **
(SyntaxError) invalid syntax found on iex:12:47:
# !> ┌─ error: iex:12:47
# !> │
# !> 12 │ %{:foo => foo, "fizz" => "buzz", "bar" => bar, fizz: :buzz, :baz
=> baz}
# !> │ ^
# !> │
# !> unexpected expression after keyword list. Keyword lists must
always come last in lists and maps.
# !>
# !> Syntax error after: ','
On Wednesday, June 28, 2023 at 10:44:44 PM UTC-5 Christopher Keele wrote:
> Posted that last reply early. continued:
>
> Part of the elegance in of making $:foo and &"bar" expand to a valid
> pair, right before Map expansion handles pairs as {:%{}, [], [...pairs]},
> is that it *could* easily allow us to support mixing tagged variable
> captures anywhere in the existing syntax constructs: This is not true of my
> prototype today, though, it would need more work based on how we decide to
> handle it:
>
> {foo, bar, baz} = {1, 2, 3}
>
> %{$:foo, "fizz" => "buzz", $"bar", fizz: :buzz}
> # => %{:fizz => :buzz, :foo => 1, "bar" => 2, "fizz" => "buzz"}
>
> %{$:foo, "fizz" => "buzz", $"bar", fizz: :buzz, $:baz} # !> **
> (SyntaxError) invalid syntax found on iex:12:47:
> # !> ┌─ error: iex:12:47
> # !> │
> # !> 12 │ %{$:foo, "fizz" => "buzz", $"bar", fizz: :buzz, $:baz}
> # !> │ ^
> # !> │
> # !> unexpected expression after keyword list. Keyword lists must
> always come last in lists and maps. Therefore, this is not allowed:
> # !>
> # !> [some: :value, :another]
> # !> %{some: :value, another => value}
> # !>
> # !> Instead, reorder it to be the last entry:
> # !>
> # !> [:another, some: :value]
> # !> %{another => value, some: :value}
> # !>
> # !> Syntax error after: ','
>
>
>
> On Wednesday, June 28, 2023 at 10:32:20 PM UTC-5 Christopher Keele wrote:
>
>> > Alternatively, the `$` 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
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>>>> To unsubscribe from this group and stop receiving emails from it, send
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>>>> 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>
>>>> .
>>>>
>>>>
>>>> --
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>>>>
>>> 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
>>>
>>
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