## 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 case to match only on a
given iteration of the loop:
```
match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
print("italics")
case (1, let x) where x.contains(.bold):
print("bold")
default: break }
}
// prints "italics"
// prints "bold"
```
## Notes / Discussion:
- Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
- Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
- FizzBuzz using proposed Swift `match` statement:
```
for i in 1...100 {
var output = ""
match 0 {
case (i % 3): output += "Fizz"
case (i % 3): output += "Buzz"
default: output = String(i)
}
print(output)
}
// `15` prints "FizzBuzz"
```
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