Ideally, <- and = would have the same semantics in "for" and "with", but
I'm not sure how confusing "for {:ok, post} = posts, do: ..." would be.
Given all that, I find "for!" to be the best approach, except for the name.
As stated before, a bang version feels like a precedent. I think "fors"
("for strict") is better (like def and defp), or maybe a totally different
name for a loop (while?, loop?, rep?, iter?).
wt., 15 cze 2021 o 19:06 José Valim <jose.va...@dashbit.co> napisał(a):
> I am against introducing a new operator because I think that will be just
> unclear. Regardless if we pick "<!-" or "<<-", I don't think the notation
> would be clear to everyone reading the code. Between for!, a strict option,
> and the operator, the operator is, in my opinion, the most unreadable.
>
> Furthermore, the operator doesn't have a use in "with", because "with" can
> use = for strict matches.
>
>
> On Tue, Jun 15, 2021 at 6:59 PM Stefan Chrobot <ste...@chrobot.io> wrote:
>
>> How about an "else" block for the items that don't match?
>>
>> for {:ok, item} <- items do
>> # ...
>> else
>> # ...
>> end
>>
>> This would be consistent with <- in "with". I'm assuming the intention
>> would be to prefer "for!" over "for" for most of the cases so the issue
>> with this is a need to figure out a terse syntax for raising a MatchError
>> without having to type something like "else _ -> raise MatchError"; plus
>> also include what was being attempted in the error message. Is "else raise"
>> an option?
>>
>> Best,
>> Stefan
>>
>> niedz., 13 cze 2021 o 14:06 Adam Lancaster <a...@a-corp.co.uk>
>> napisał(a):
>>
>>> That makes sense!
>>>
>>> I guess you could make a new operator available outside of the `for`
>>> too, like back in a `with`... Maybe `<-!`
>>>
>>> Best
>>>
>>> Adam
>>>
>>>
>>> On 11 Jun 2021, at 19:11, Paul Schoenfelder <
>>> paulschoenfel...@fastmail.com> wrote:
>>>
>>> In my opinion, internal consistency is part of the mental model, so
>>> inconsistency reflects a flaw in the model. That said, I think that's
>>> probably talking past you on this a bit, and I get what your point is:
>>> ultimately if it is reasonably intuitive, consistency can be allowed to
>>> fall by the wayside a bit. I guess where I disagree is that I'm not sure
>>> this will intuitive for someone not already steeped in the language. The
>>> point I was getting at by comparing `for` and `with` is that they both make
>>> use of the same `<-` operator in a way that is consistent across both
>>> forms, but with `for!` that falls apart.
>>>
>>> Now back to `for!`. Even though it looks just like `for`, the `<-`
>>> operator starts to behave like `=`. If you are skimming code and happen to
>>> miss the single character difference between the two (`for` vs `for!`), you
>>> will wind up with a very different idea about what the same code does. The
>>> human brain is terrible at distinguishing small differences like this, it's
>>> why you can typo things like `behavior` and `behaviour` and read right over
>>> it without noticing, sometimes even when you are _trying_ to notice those
>>> things.
>>>
>>> I think it would be far better for us to use a new operator in place of
>>> `<-`, rather than a new special form that looks basically identical to an
>>> existing one, but works differently in subtle ways. Not to mention, the
>>> operator approach would allow one to mix both `<-` and the new operator
>>> together in the same `for`, should it be useful to do so. In any case, I
>>> don't really have a strong opinion on what that operator is specifically,
>>> but I am much more in favor of that direction, than I am `for!`.
>>>
>>> Paul
>>>
>>>
>>> On Fri, Jun 11, 2021, at 9:24 AM, Adam Lancaster wrote:
>>>
>>> I'm definitely sympathetic to that idea.
>>>
>>> I think part of what internal consistency buys us is predictability and
>>> therefore a quicker path to a good mental model about what the code is
>>> going to do. But the mental model is the more important thing. Which is
>>> just to say if we don't have internal consistency but we can get to a good
>>> mental model, then I think it might be okay.
>>>
>>> I think given other functions that follow the same idea, seeing a `for!`
>>> would certainly communicate "right this is expected to raise under some
>>> condition" - at least to me.
>>>
>>> There's also not an obvious way to have `for` mimic `with` when I think
>>> about it because say you do this:
>>>
>>> ```
>>> for [a, _] = [1, 2], do: ...
>>> ```
>>>
>>> there is no way to distinguish it from a filter - where `=` should not
>>> raise a match error.
>>>
>>> I think you'd have to more clearly de-mark the difference between the
>>> generators and the filters, which feels like a big change.
>>>
>>>
>>> Best
>>>
>>> Adam
>>>
>>>
>>>
>>>
>>> On 11 Jun 2021, at 00:13, Paul Schoenfelder <
>>> paulschoenfel...@fastmail.com> wrote:
>>>
>>> I’m generally in favor of the option to have stricter semantics, but to
>>> me the introduction of `for!` feels out of sync with other special forms,
>>> none of which are bang-form. Furthermore, especially in contrast to `with`,
>>> you end up with this weird dichotomy with the `<-` operator, where
>>> sometimes it means a filtering match, and other times where it means strict
>>> match. That kind of syntactical inconsistency in a language feels like a
>>> bad precedent to set, despite what feels like a reasonable compromise. It’s
>>> also notable to me that there are easy ways to program defensively to force
>>> match errors if you want them, within the current syntax, but obviously
>>> that comes at the cost of more verbosity.
>>>
>>> I’m not sure what the right answer is, but this feels to me like rushing
>>> to solve a specific problem without spending enough time considering how it
>>> meshes with the rest of the language in terms of cognitive complexity,
>>> particularly for those new to the language.
>>>
>>> Anyway, that’s my two cents. I’m a fan of the concept for sure, but
>>> would almost prefer to see the semantics changed in a major version bump,
>>> to match `with`, even if that meant manually updating a bunch of my code,
>>> because at least it keeps the language self consistent. I’ll admit I’m
>>> probably an outlier on that though.
>>>
>>> Paul
>>>
>>> On Thu, Jun 10, 2021, at 6:16 PM, Christopher Keele wrote:
>>>
>>> That's fair enough! Though from my perspective both for! and strict:
>>> true would be about equally far from the <- where matches fail. But I
>>> can see the keyword format getting lost in the filters and other keywords.
>>>
>>> On Thu, Jun 10, 2021 at 3:14 PM José Valim <jose.va...@dashbit.co>
>>> wrote:
>>>
>>> Sorry, I meant to someone reading the code. The strict option is
>>> modifying the behavior of the operator <-, which may be quite before it in
>>> the text.
>>>
>>> I prefer for! in this case as it is upfront.
>>>
>>> On Fri, Jun 11, 2021 at 00:09 Christopher Keele <christheke...@gmail.com>
>>> wrote:
>>>
>>> > My concern with :strict is that it changes the behavior considerably
>>> of the generators but it may show up only quite later on, far from them,
>>> especially if you have multiple filters.
>>>
>>> Could you elaborate? I don't quite think I understand, particularly *"[the
>>> behaviour] may show up only quite later on"*
>>>
>>> Does "quite later" here refer to code distance (the MatchError's
>>> stacktrace would point away from/bury the for location)? Or temporal
>>> distance?
>>>
>>> On Thursday, June 10, 2021 at 2:58:03 PM UTC-7 José Valim wrote:
>>>
>>> My concern with :strict is that it changes the behavior considerably of
>>> the generators but it may show up only quite later on, far from them,
>>> especially if you have multiple filters.
>>>
>>>
>>> On Thu, Jun 10, 2021 at 23:56 Christopher Keele <christ...@gmail.com>
>>> wrote:
>>>
>>> > for {:ok, num} <- list, strict: true, do: num
>>>
>>> Agreed, this is more or less exactly what I was pitching.
>>> On Wednesday, June 9, 2021 at 10:16:25 PM UTC-7 tal...@gmail.com wrote:
>>>
>>> I would like to add a solution within the existing language:
>>>
>>>
>>> ```elixir
>>>
>>> > list = [{:ok, 1}, {:ok, 2}, {:error, :fail}, {:ok, 4}]
>>> > for el <- list, do: ({:ok, num} = el; num)
>>> ** (MatchError) no match of right hand side value: {:error, :fail}
>>> ```
>>> I think this is reasonable.
>>>
>>> Acctually the built in filtering in `for` caught me off guard, I was
>>> expecting for to fail unless all elements matched. So for me the better
>>> solution would be to always make matching in `for` strict. But I guess this
>>> is too late now for backwards compatibility. Another alternative to `for!`
>>> would be:
>>>
>>> ```elixir
>>>
>>> > list = [{:ok, 1}, {:ok, 2}, {:error, :fail}, {:ok, 4}]
>>> > for {:ok, num} <- list, strict: true, do: num
>>> ** (MatchError) no match of right hand side value: {:error, :fail}
>>> ```
>>>
>>> I don't like the use of the exclamation mark in `for!` because it has
>>> little meaning relative to the existing use of the exclamation mark in
>>> Elixir.
>>>
>>> onsdag 9. juni 2021 kl. 13:17:04 UTC+2 skrev ad...@a-corp.co.uk:
>>>
>>> I also love the proposal.
>>>
>>> It's a shame we can't re-use the `with` semantics of `=` raising a match
>>> error in the for.
>>>
>>> My two cents is `for!` makes the most sense, and follows the conventions
>>> of other functions.
>>>
>>> Best
>>>
>>> Adam
>>>
>>>
>>> On 8 Jun 2021, at 18:18, Christopher Keele <christ...@gmail.com> wrote:
>>>
>>> This feature would be very useful, I've experience this signature-change
>>> pain point before too (and kind of have been avoiding `for` ever since,
>>> TBH).
>>>
>>> I'm reluctant to increase the surface area of the language itself, what
>>> do you think about adding a `:strict` option to `for` instead of a new
>>> special form/kernel macro/operator?
>>> On Monday, June 7, 2021 at 9:50:45 AM UTC-7 eric.meado...@gmail.com
>>> wrote:
>>>
>>> ## Background
>>>
>>> `for` comprehensions are one of the most powerful features in Elixir. It
>>> supports both enumerable and bitstring generators, filters through boolean
>>> expressions and pattern matching, collectibles with `:into` and folding
>>> with `:reduce`.
>>>
>>> One of the features are automatic filtering by patterns in generators:
>>>
>>> ```elixir
>>> list = [{:ok, 1}, {:ok, 2}, {:error, :fail}, {:ok, 4}]
>>> for {:ok, num} <- list, do: num
>>> #=> [1, 2, 4]
>>> ```
>>>
>>> Generator filtering is very powerful because it allows you to succinctly
>>> filter out data that is not relevant to the comprehension in the same
>>> expression that you are generating elements out of your
>>> enumerable/bitstrings. But the implicit filtering can be dangerous because
>>> changes in the shape of the data will silently be removed which can cause
>>> hard to catch bugs.
>>>
>>> The following example can show how this can be an issue when testing
>>> `Posts.create/0`. If a change causes the function to start returning `{:ok,
>>> %Post{}}` instead of the expected `%Post{}` the test will pass even though
>>> we have a bug.
>>>
>>> ```elixir
>>> test "create posts" do
>>> posts = Posts.create()
>>> for %Post{id: id} <- posts, do: assert is_integer(id)
>>> end
>>> ```
>>>
>>> The example uses a test to highlight the issue but it can just as well
>>> happen in production code, specially when refactoring in other parts of the
>>> code base than the comprehension.
>>>
>>> Elixir is a dynamically typed language but dynamic typing errors are
>>> less of an issue compared to many other dynamic languages because we are
>>> usual strict in the data we accept by using pattern matching and guard
>>> functions. `for` is by design not strict on the shape of data it accepts
>>> and therefor loses the nice property of early failure on incorrect data.
>>>
>>> ## Proposal
>>>
>>> I propose an alternative comprehension macro called `for!` that has the
>>> same functionality as `for` but instead of filtering on patterns in
>>> generators it will raise a `MatchError`.
>>>
>>> ```elixir
>>> posts = [{:ok, %Post{}}]
>>> for! %Post{id: id} <- posts, do: assert is_integer(id)
>>> #=> ** (MatchError) no match of right hand side value: {:ok, %Post{}}
>>> ```
>>>
>>> Pattern matching when not generating values with `=` remains unchanged.
>>>
>>> `for!` gives the developer an option to be strict on the data it accepts
>>> instead of silently ignoring data that does not match.
>>>
>>> ## Other considerations
>>>
>>> You can get strict matching with `for` today by first assigning to a
>>> variable. This way you can also mix filtering and strict matching patterns.
>>>
>>> ```elixir
>>> posts = [{:ok, %Post{}}]
>>> for post <- posts,
>>> %Post{id: id} = post,
>>> do: assert is_integer(id)
>>> #=> ** (MatchError) no match of right hand side value: {:ok, %Post{}}
>>> ```
>>>
>>> Another alternative is to introduce a new operator such as `<<-` (the
>>> actual token can be anything, `<<-` is only used as an example) for raising
>>> pattern matches instead of introducing a completely new macro.
>>>
>>> ```elixir
>>> posts = [{:ok, %Post{}}]
>>> for %Post{id: id} <<- posts, do: assert is_integer(id)
>>> #=> ** (MatchError) no match of right hand side value: {:ok, %Post{}}
>>> ```
>>>
>>> A downside of adding new functions or macros is that it doesn't compose
>>> as well compared to adding options (or operators) to existing functions. If
>>> we want to add another variant of comprehensions in the future we might be
>>> in the position that we need 4 macros, and then 8 and so on.
>>>
>>> Another benefit of adding an operator is that you can mix both `<-` and
>>> `<<-` in a single comprehension.
>>>
>>> The downside of an operator is that it adds more complexity for the
>>> language user. We would also need an operator that is visually close to
>>> `<-` but still distinctive enough that they are easy to separate since
>>> their behavior are very difference.
>>>
>>>
>>> --
>>> 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/42adcfba-12d8-4469-a156-f412b0d290a9n%40googlegroups.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/42adcfba-12d8-4469-a156-f412b0d290a9n%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/f4d5c0be-567a-4a7d-9b39-68202226c788n%40googlegroups.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/f4d5c0be-567a-4a7d-9b39-68202226c788n%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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/0ce03abc-61bb-4423-b6a8-704d1d62169fn%40googlegroups.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/0ce03abc-61bb-4423-b6a8-704d1d62169fn%40googlegroups.com?utm_medium=email&utm_source=footer>
>>> .
>>>
>>>
>>> --
>>> You received this message because you are subscribed to a topic in the
>>> Google Groups "elixir-lang-core" group.
>>> To unsubscribe from this topic, visit
>>> https://groups.google.com/d/topic/elixir-lang-core/LEUD2alHPiE/unsubscribe
>>> .
>>> To unsubscribe from this group and all its topics, 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/CAGnRm4K01hBRkjLaRPj5ktViNNjYqdFbKdysvFcDVG%3DgBp78dA%40mail.gmail.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4K01hBRkjLaRPj5ktViNNjYqdFbKdysvFcDVG%3DgBp78dA%40mail.gmail.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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/CAD9kT2QPn_prFiS%2BR9eemqA43DMvvOB8NrAweL2PgE_ZR2g6Cg%40mail.gmail.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/CAD9kT2QPn_prFiS%2BR9eemqA43DMvvOB8NrAweL2PgE_ZR2g6Cg%40mail.gmail.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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/8a6cf634-cda5-4445-8230-4b7b69ed5ca8%40www.fastmail.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/8a6cf634-cda5-4445-8230-4b7b69ed5ca8%40www.fastmail.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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/65609056-4A25-45FA-B91F-84D4DF292129%40a-corp.co.uk
>>> <https://groups.google.com/d/msgid/elixir-lang-core/65609056-4A25-45FA-B91F-84D4DF292129%40a-corp.co.uk?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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/4f04033a-d509-460e-8205-ad23e1251b1e%40www.fastmail.com
>>> <https://groups.google.com/d/msgid/elixir-lang-core/4f04033a-d509-460e-8205-ad23e1251b1e%40www.fastmail.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-core+unsubscr...@googlegroups.com.
>>> To view this discussion on the web visit
>>> https://groups.google.com/d/msgid/elixir-lang-core/CE301440-828B-41A5-B388-75CD6FF94699%40a-corp.co.uk
>>> <https://groups.google.com/d/msgid/elixir-lang-core/CE301440-828B-41A5-B388-75CD6FF94699%40a-corp.co.uk?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-core+unsubscr...@googlegroups.com.
>> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/elixir-lang-core/CACzMe7b6srxk_vE45h-qY--g61t-Lzqgvix46jX2GxQRE46FGA%40mail.gmail.com
>> <https://groups.google.com/d/msgid/elixir-lang-core/CACzMe7b6srxk_vE45h-qY--g61t-Lzqgvix46jX2GxQRE46FGA%40mail.gmail.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-core+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KFQ_2BNCXmFdJuxJC2XWD-hSpbod5_wt%2Bka%3DSBpfFcpg%40mail.gmail.com
> <https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KFQ_2BNCXmFdJuxJC2XWD-hSpbod5_wt%2Bka%3DSBpfFcpg%40mail.gmail.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-core+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/elixir-lang-core/CACzMe7YNuuhqq9S%2BJu6W_829XVBbPZar9vj5pAjkcan99NO82w%40mail.gmail.com.
