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SIP-62 - For comprehension improvements (#20522)
Implementation for SIP-62. ### Summary of the changes For more details read the committed markdown file here: scala/improvement-proposals#79 This introduces improvements to `for` comprehensions in Scala to improve usability and simplify desugaring. The changes are hidden behind a language import `scala.language.experimental.betterFors`. The main changes are: 1. **Starting `for` comprehensions with aliases**: - **Current Syntax**: ```scala val a = 1 for { b <- Some(2) c <- doSth(a) } yield b + c ``` - **New Syntax**: ```scala for { a = 1 b <- Some(2) c <- doSth(a) } yield b + c ``` 2. **Simpler Desugaring for Pure Aliases**: - **Current Desugaring**: ```scala for { a <- doSth(arg) b = a } yield a + b ``` Desugars to: ```scala doSth(arg).map { a => val b = a (a, b) }.map { case (a, b) => a + b } ``` - **New Desugaring**: (where possible) ```scala doSth(arg).map { a => val b = a a + b } ``` 3. **Avoiding Redundant `map` Calls**: - **Current Desugaring**: ```scala for { a <- List(1, 2, 3) } yield a ``` Desugars to: ```scala List(1, 2, 3).map(a => a) ``` - **New Desugaring**: ```scala List(1, 2, 3) ```
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,12 @@ | ||
| List((1,3), (1,4), (2,3), (2,4)) | ||
| List((1,2,3), (1,2,4)) | ||
| List((1,3), (1,4), (2,3), (2,4)) | ||
| List((2,3), (2,4)) | ||
| List((2,3), (2,4)) | ||
| List((1,2), (2,4)) | ||
| List(1, 2, 3) | ||
| List((2,3,6)) | ||
| List(6) | ||
| List(3, 6) | ||
| List(6) | ||
| List(2) |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,105 @@ | ||
| import scala.language.experimental.betterFors | ||
|
|
||
| def for1 = | ||
| for { | ||
| a = 1 | ||
| b <- List(a, 2) | ||
| c <- List(3, 4) | ||
| } yield (b, c) | ||
|
|
||
| def for2 = | ||
| for | ||
| a = 1 | ||
| b = 2 | ||
| c <- List(3, 4) | ||
| yield (a, b, c) | ||
|
|
||
| def for3 = | ||
| for { | ||
| a = 1 | ||
| b <- List(a, 2) | ||
| c = 3 | ||
| d <- List(c, 4) | ||
| } yield (b, d) | ||
|
|
||
| def for4 = | ||
| for { | ||
| a = 1 | ||
| b <- List(a, 2) | ||
| if b > 1 | ||
| c <- List(3, 4) | ||
| } yield (b, c) | ||
|
|
||
| def for5 = | ||
| for { | ||
| a = 1 | ||
| b <- List(a, 2) | ||
| c = 3 | ||
| if b > 1 | ||
| d <- List(c, 4) | ||
| } yield (b, d) | ||
|
|
||
| def for6 = | ||
| for { | ||
| a = 1 | ||
| b = 2 | ||
| c <- for { | ||
| x <- List(a, b) | ||
| y = x * 2 | ||
| } yield (x, y) | ||
| } yield c | ||
|
|
||
| def for7 = | ||
| for { | ||
| a <- List(1, 2, 3) | ||
| } yield a | ||
|
|
||
| def for8 = | ||
| for { | ||
| a <- List(1, 2) | ||
| b = a + 1 | ||
| if b > 2 | ||
| c = b * 2 | ||
| if c < 8 | ||
| } yield (a, b, c) | ||
|
|
||
| def for9 = | ||
| for { | ||
| a <- List(1, 2) | ||
| b = a * 2 | ||
| if b > 2 | ||
| } yield a + b | ||
|
|
||
| def for10 = | ||
| for { | ||
| a <- List(1, 2) | ||
| b = a * 2 | ||
| } yield a + b | ||
|
|
||
| def for11 = | ||
| for { | ||
| a <- List(1, 2) | ||
| b = a * 2 | ||
| if b > 2 && b % 2 == 0 | ||
| } yield a + b | ||
|
|
||
| def for12 = | ||
| for { | ||
| a <- List(1, 2) | ||
| if a > 1 | ||
| } yield a | ||
|
|
||
| object Test extends App { | ||
| println(for1) | ||
| println(for2) | ||
| println(for3) | ||
| println(for4) | ||
| println(for5) | ||
| println(for6) | ||
| println(for7) | ||
| println(for8) | ||
| println(for9) | ||
| println(for10) | ||
| println(for11) | ||
| println(for12) | ||
| } |
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