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?*
>>>
>>>
>>> --
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>>>
>>>
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>>> .
>>>
>>
>>
>> --
>> Austin Ziegler • halos...@gmail.com • aus...@halostatue.ca
>> http://www.halostatue.ca/ • http://twitter.com/halostatue
>>
>
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