Re: [swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
A high bar for new syntax is fair and expected, and by posting I was hoping
to maybe find an alternative in the comments here.
But AFAIK there's currently no ability in Swift to:
"Evaluate a *single* control expression against all of these patterns, and
execute any and all cases that match"
Multiple `if-case` statements, each re-stating the control expression, are
ok.
But that's definitely not as clear or concise as a switch-like construct
with the single control expression at the top. Or perhaps some other
alternative such as the mentioned `continue` or somehow enumerating a set
of cases.
On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu wrote:
> Robert is quite right--I'm not sure what we're designing for here. There's
> a very high bar for introducing new syntax and a distaste for the existing
> syntax is not a motivating use case.
>
>
> On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> There have been earlier suggestions for an alternative to `fallthrough`
>> that would continue matching cases; I think that is much more likely to get
>> support than a whole new construct with only a subtle difference from an
>> existing one—would that be an acceptable alternative to you?
>>
>> > On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <
>> swift-evolution@swift.org> wrote:
>> >
>> > ## Title
>> >
>> > Add `match` statement as `switch`-like syntax alternative to `if case`
>> pattern matching
>> >
>> > ## Summary:
>> >
>> > The syntax of the `switch` statement is familiar, succinct, elegant,
>> and understandable. Swift pattern-matching tutorials use `switch`
>> statements almost exclusively, with small sections at the end for
>> alternatives such as `if case`.
>> >
>> > However, the `switch` statement has several unique behaviors unrelated
>> to pattern matching. Namely:
>> >
>> > - Only the *first* matching case is executed. Subsequent matching
>> cases are not executed.
>> > - `default:` case is required, even for expressions where a default
>> case does not make sense.
>> >
>> > These behaviors prevent `switch` from being used as a generic
>> match-patterns-against-a-single-expression statement.
>> >
>> > Swift should contain an equally-good pattern-matching statement that
>> does not limit itself single-branch switching.
>> >
>> > ## Pitch:
>> >
>> > Add a `match` statement with the same elegant syntax as the `switch`
>> statement, but without any of the "branch switching" baggage.
>> >
>> > ```
>> > match someValue {
>> > case patternOne:
>> > always executed if pattern matches
>> > case patternTwo:
>> > always executed if pattern matches
>> > }
>> > ```
>> >
>> > The match statement would allow a single value to be filtered through
>> *multiple* cases of pattern-matching evaluation.
>> >
>> > ## Example:
>> >
>> > ```
>> > struct TextFlags: OptionSet {
>> > let rawValue: Int
>> > static let italics = TextFlags(rawValue: 1 << 1)
>> > static let bold= TextFlags(rawValue: 1 << 2)
>> > }
>> >
>> > let textFlags: TextFlags = [.italics, .bold]
>> >
>> >
>> >
>> > // SWITCH STATEMENT
>> > switch textFlags {
>> > case let x where x.contains(.italics):
>> > print("italics")
>> > case let x where x.contains(.bold):
>> > print("bold")
>> > default:
>> > print("forced to include a default case")
>> > }
>> > // prints "italics"
>> > // Does NOT print "bold", despite .bold being set.
>> >
>> >
>> >
>> > // MATCH STATEMENT
>> > match textFlags {
>> > case let x where x.contains(.italics):
>> > print("italics")
>> > case let x where x.contains(.bold):
>> > print("bold")
>> > }
>> > // prints "italics"
>> > // prints "bold"
>> > ```
>> >
>> > ## Enum vs. OptionSet
>> >
>> > The basic difference between `switch` and `match` is the same
>> conceptual difference between `Emum` and an `OptionSet` bitmask.
>> >
>> > `switch` is essentially designed for enums: switching to a single
>> logical branch based on the single distinct ca
Re: [swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
I understand your point about multiple cases + the halting problem when applied to the implementation details of switch and pattern matching. I think where we're missing is that what I'm envisioning would be syntactic sugar that would be de-sugared into a a line of single-case `if-case` statements. It would have nothing to do with matching multiple cases via the same mechanism as `switch`. Motivation for the change vs. a line of if-case statements is a valid question of course :) On Sat, Nov 18, 2017 at 10:48 AM, Robert Widmann wrote: > > On Nov 18, 2017, at 4:42 AM, Peter Kamb wrote: > > Thanks for the review. > > Motivation is essentially to provide a low-friction way to write `if case > / if case / if case` statements that are all matching on the same > expression, much in the same way that `switch` offers a way to write `if / > else if / else` statements against the same expression. > > > That much I understand, but there’s no motivation as to why this kind of > change needs to occur over a line of if-case statements. There isn’t > anything in the pitch that demonstrates friction outside of a personal > distaste for the syntax. > > > > > Only the *first* matching case is executed. Subsequent matching cases > are not executed. > > This is not unrelated to pattern matching, this is the expected > behavior of every pattern matching algorithm. > > By that I meant that a string of `if case / if case / if case` statements, > each individually matching against the same expression, would match and > execute all 3 cases. A `switch` using the same 3 cases would execute only > the first matching case. I took the "only first case" behavior to be a > property of the *switch*, not of "pattern matching" in general? But perhaps > I'm using the wrong terminology. > > > > Allow filtering a single object through *multiple* cases of pattern > matching, executing *all* cases that match. > > Again, this is not pattern matching. > > A switch can have multiple cases that "would" match and execute, if you > were to comment out the successful case above them. Why would a > hypothetical statement that executed *all* matching cases, rather than just > the first case, not be pattern matching? It wouldn't be a `switch`, for > sure, but it seems like it would be just as much "pattern matching" as an > `if-case`. > > > It is not pattern matching because it violates minimality - and we have a > warning today when you write switches of overlapping patterns. Consider > the semantics of pattern matching in the context of a tree - but it just so > happens the nodes of this tree are your patterns and the branches, actual > branches where control flow splits along the case-matrix. The goal is not > to execute a find-all, the goal is to execute a find - exactly like a > regular expression. > > > > This is not boilerplate! > > An `if case / if case` pair would not have the same concept of a shared > `else/default` case used by a `switch` or `if/else`, which is why I said it > would not be needed. `if-case` does not require an else/default. (Perhaps > boilerplate was the wrong word, and I definitely didn't mean to suggest > that switches should no longer require `default:`). > > > You misunderstand me. This is the same problem as the one I brought up > before: You are subject to the halting problem in the general case. The > catch-all clause isn’t just noise, it’s a fundamental necessity to > guarantee sound semantics for this particular class of switch statements > that, according to a fundamental barrier in computer science, cannot be > checked for exhaustiveness. > > On top of that, we cannot allow you to write an uncovered switch statement > full of expression patterns because you would most-assuredly not handle all > possible cases (suppose I extend your OptionSet with a new bit-pattern in a > different module - your code will miscompile). > > > On Sat, Nov 18, 2017 at 12:19 AM, Robert Widmann > wrote: > >> Having spent a lot of time with ‘switch’, I don’t understand any of the >> motivations or corresponding justifications for this idea. Comments inline: >> >> ~Robert Widmann >> >> 2017/11/17 15:06、Peter Kamb via swift-evolution < >> swift-evolution@swift.org>のメール: >> >> ## Title >> >> Add `match` statement as `switch`-like syntax alternative to `if case` >> pattern matching >> >> ## Summary: >> >> The syntax of the `switch` statement is familiar, succinct, elegant, and >> understandable. Swift pattern-matching tutorials use `switch` statements >> almost exclusively, with small sections at the end f
Re: [swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
> alternative to `fallthrough` that would continue matching cases
Yes, that would be interesting to look into. Do you have any references or
remember what the proposed keyword was called? Do any other languages have
this feature?
On Sat, Nov 18, 2017 at 10:53 AM Kevin Nattinger
wrote:
> There have been earlier suggestions for an alternative to `fallthrough`
> that would continue matching cases; I think that is much more likely to get
> support than a whole new construct with only a subtle difference from an
> existing one—would that be an acceptable alternative to you?
>
> > On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <
> swift-evolution@swift.org> wrote:
> >
> > ## Title
> >
> > Add `match` statement as `switch`-like syntax alternative to `if case`
> pattern matching
> >
> > ## Summary:
> >
> > The syntax of the `switch` statement is familiar, succinct, elegant, and
> understandable. Swift pattern-matching tutorials use `switch` statements
> almost exclusively, with small sections at the end for alternatives such as
> `if case`.
> >
> > However, the `switch` statement has several unique behaviors unrelated
> to pattern matching. Namely:
> >
> > - Only the *first* matching case is executed. Subsequent matching cases
> are not executed.
> > - `default:` case is required, even for expressions where a default
> case does not make sense.
> >
> > These behaviors prevent `switch` from being used as a generic
> match-patterns-against-a-single-expression statement.
> >
> > Swift should contain an equally-good pattern-matching statement that
> does not limit itself single-branch switching.
> >
> > ## Pitch:
> >
> > Add a `match` statement with the same elegant syntax as the `switch`
> statement, but without any of the "branch switching" baggage.
> >
> > ```
> > match someValue {
> > case patternOne:
> > always executed if pattern matches
> > case patternTwo:
> > always executed if pattern matches
> > }
> > ```
> >
> > The match statement would allow a single value to be filtered through
> *multiple* cases of pattern-matching evaluation.
> >
> > ## Example:
> >
> > ```
> > struct TextFlags: OptionSet {
> > let rawValue: Int
> > static let italics = TextFlags(rawValue: 1 << 1)
> > static let bold= TextFlags(rawValue: 1 << 2)
> > }
> >
> > let textFlags: TextFlags = [.italics, .bold]
> >
> >
> >
> > // SWITCH STATEMENT
> > switch textFlags {
> > case let x where x.contains(.italics):
> > print("italics")
> > case let x where x.contains(.bold):
> > print("bold")
> > default:
> > print("forced to include a default case")
> > }
> > // prints "italics"
> > // Does NOT print "bold", despite .bold being set.
> >
> >
> >
> > // MATCH STATEMENT
> > match textFlags {
> > case let x where x.contains(.italics):
> > print("italics")
> > case let x where x.contains(.bold):
> > print("bold")
> > }
> > // prints "italics"
> > // prints "bold"
> > ```
> >
> > ## Enum vs. OptionSet
> >
> > The basic difference between `switch` and `match` is the same conceptual
> difference between `Emum` and an `OptionSet` bitmask.
> >
> > `switch` is essentially designed for enums: switching to a single
> logical branch based on the single distinct case represented by the enum.
> >
> > `match` would be designed for OptionSet bitmasks and similar constructs.
> Executing behavior for *any and all* of the following cases and patterns
> that match.
> >
> > The programmer would choose between `switch` or `match` based on the
> goal of the pattern matching. For example, pattern matching a String.
> `switch` would be appropriate for evaluating a String that represents the
> rawValue of an enum. But `match` would be more appropriate for evaluating a
> single input String against multiple unrelated-to-each-other regexes.
> >
> > ## Existing Alternatives
> >
> > `switch` cannot be used to match multiple cases. There are several ways
> "test a value against multiple patterns, executing behavior for each
> pattern that matches", but none are as elegant and understandable as the
> switch statement syntax.
> >
> > Example using a string of independent `if case` statements:
> >
> > ```
> > if case let x = textFlags, x.contains(.italics) {
> >
Re: [swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
Thanks for the review.
Motivation is essentially to provide a low-friction way to write `if case /
if case / if case` statements that are all matching on the same expression,
much in the same way that `switch` offers a way to write `if / else if /
else` statements against the same expression.
> > Only the *first* matching case is executed. Subsequent matching cases
are not executed.
> This is not unrelated to pattern matching, this is the expected behavior
of every pattern matching algorithm.
By that I meant that a string of `if case / if case / if case` statements,
each individually matching against the same expression, would match and
execute all 3 cases. A `switch` using the same 3 cases would execute only
the first matching case. I took the "only first case" behavior to be a
property of the *switch*, not of "pattern matching" in general? But perhaps
I'm using the wrong terminology.
> > Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
> Again, this is not pattern matching.
A switch can have multiple cases that "would" match and execute, if you
were to comment out the successful case above them. Why would a
hypothetical statement that executed *all* matching cases, rather than just
the first case, not be pattern matching? It wouldn't be a `switch`, for
sure, but it seems like it would be just as much "pattern matching" as an
`if-case`.
> This is not boilerplate!
An `if case / if case` pair would not have the same concept of a shared
`else/default` case used by a `switch` or `if/else`, which is why I said it
would not be needed. `if-case` does not require an else/default. (Perhaps
boilerplate was the wrong word, and I definitely didn't mean to suggest
that switches should no longer require `default:`).
> A syntax that duplicates the existing if-case construct
Right, it would duplicate `if-case` just as `switch` duplicates `if-else`.
Cheers, thanks.
Peter
On Sat, Nov 18, 2017 at 12:19 AM, Robert Widmann
wrote:
> Having spent a lot of time with ‘switch’, I don’t understand any of the
> motivations or corresponding justifications for this idea. Comments inline:
>
> ~Robert Widmann
>
> 2017/11/17 15:06、Peter Kamb via swift-evolution >のメール:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case`
> pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and
> understandable. Swift pattern-matching tutorials use `switch` statements
> almost exclusively, with small sections at the end for alternatives such as
> `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated to
> pattern matching. Namely:
>
>
>
>
> - Only the *first* matching case is executed. Subsequent matching cases
> are not executed.
>
>
> This is not unrelated to pattern matching, this is the expected behavior
> of every pattern matching algorithm. The intent is to compile a
> switch-case tree down to (conceptually) a (hopefully minimal) if-else
> tree. You may be thinking of the behavior of switch in C and C-likes which
> is most certainly not pattern matching and includes behavior Swift has
> explicitly chosen to avoid like implicit fallthroughs.
>
> - `default:` case is required, even for expressions where a default case
> does not make sense.
>
>
> Expression patterns may look to be covered “at first glance”, but the
> analysis required to prove that is equivalent to solving the halting
> problem in the general case. Further, your proposed idea has absolutely
> nothing to do with this.
>
>
> These behaviors prevent `switch` from being used as a generic
> match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that does
> not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
> statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
> *multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold= TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
>
[swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
## Title
Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching
## Summary:
The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.
However, the `switch` statement has several unique behaviors unrelated to
pattern matching. Namely:
- Only the *first* matching case is executed. Subsequent matching cases
are not executed.
- `default:` case is required, even for expressions where a default case
does not make sense.
These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
Swift should contain an equally-good pattern-matching statement that does
not limit itself single-branch switching.
## Pitch:
Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
```
match someValue {
case patternOne:
always executed if pattern matches
case patternTwo:
always executed if pattern matches
}
```
The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
## Example:
```
struct TextFlags: OptionSet {
let rawValue: Int
static let italics = TextFlags(rawValue: 1 << 1)
static let bold= TextFlags(rawValue: 1 << 2)
}
let textFlags: TextFlags = [.italics, .bold]
// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
print("italics")
case let x where x.contains(.bold):
print("bold")
default:
print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.
// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
print("italics")
case let x where x.contains(.bold):
print("bold")
}
// prints "italics"
// prints "bold"
```
## Enum vs. OptionSet
The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.
`switch` is essentially designed for enums: switching to a single logical
branch based on the single distinct case represented by the enum.
`match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.
The programmer would choose between `switch` or `match` based on the goal
of the pattern matching. For example, pattern matching a String. `switch`
would be appropriate for evaluating a String that represents the rawValue
of an enum. But `match` would be more appropriate for evaluating a single
input String against multiple unrelated-to-each-other regexes.
## Existing Alternatives
`switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
Example using a string of independent `if case` statements:
```
if case let x = textFlags, x.contains(.italics) {
print("italics")
}
if case let x = textFlags, x.contains(.bold) {
print("bold")
}
```
## `match` statement benefits:
- Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
- A syntax that exactly aligns with the familiar, succinct, elegant, and
understandable `switch` syntax.
- The keyword "match" highlights that pattern matching will occur. Would be
even better than `switch` for initial introductions to pattern-matching.
- No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`
- Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
- A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
- Duplicated `controlExpression` is an opportunity for bugs such as typos
or changes to the expression being evaluated in a *single* `if case` from
the set, rather than all cases.
- Reduces to a pretty elegant single-case. This one-liner is an easy "just
delete whitespace" conversion from standard multi-line switch/match syntax,
whereas `if case` is not.
```
match value { case pattern:
print("matched")
}
```
- Eliminate the boilerplate `default: break` case line for non-exhaustible
expressions. Pretty much any non-Enum type being evaluated is
non-exhaustible. (This is not the *main* goal of this proposal.)
## Prototype
A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each cas
