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.
>>
>>
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