SIP-62 - For comprehension improvements

content/better-fors.md

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+---
+layout: sip
+permalink: /sips/:title.html
+stage: design
+status: submitted
+title: SIP-62 - For comprehension improvements
+---
+
+**By: Kacper Korban (VirtusLab)**
+
+## History
+
+| Date          | Version            |
+|---------------|--------------------|
+| June 6th 2023 | Initial Draft      |
+| Feb 15th 2024 | Reviewed Version   |
+
+## Summary
+
+`for`-comprehensions in Scala 3 improved their usability in comparison to Scala 2, but there are still some pain points relating both usability of `for`-comprehensions and simplicity of their desugaring.
+
+This SIP tries to address some of those problems, by changing the specification of `for`-comprehensions. From user perspective, the biggest change is allowing aliases at the start of the `for`-comprehensions. e.g.
+
+```
+for {
+  x = 1
+  y <- Some(2)
+} yield x + y
+```
+
+## Motivation
+
+There are some clear pain points related to Scala'3 `for`-comprehensions and those can be divided into two categories:
+
+1. User-facing and code simplicity problems
+
+    Specifically, for the following example written in a Haskell-style do-comprehension
+
+    ```haskell
+    do
+      a = largeExpr(arg)
+      b <- doSth(a)
+      combineM(a, b)
+    ```
+    in Scala we would have to write
+
+    ```scala
+    val a = largeExpr(b)
+    for
+      b <- doSth(a)
+      x <- combineM(a, b)
+      yield x
+    ```
+
+    This complicates the code, even in this simple example.
+2. The simplicity of desugared code
+
+    The second pain point is that the desugared code of `for`-comprehensions can often be surprisingly complicated.
+
+    e.g.
+    ```scala
+    for
+      a <- doSth(arg)
+      b = a
+    yield a + b
+    ```
+
+    Intuition would suggest for the desugared code will be of the form
+
+    ```scala
+    doSth(arg).map { a =>
+      val b = a
+      a + b
+    }
+    ```
+
+    But because of the possibility of an `if` guard being immediately after the pure alias, the desugared code is of the form
+
+    ```scala
+    doSth(arg).map { a =>
+      val b = a
+      (a, b)
+    }.map { case (a, b) =>
+      a + b
+    }
+    ```
+
+    These unnecessary assignments and additional function calls not only add unnecessary runtime overhead but can also block other optimizations from being performed.
+
+## Proposed solution
+
+This SIP suggests the following changes to `for` comprehensions:
+
+1. Allow `for` comprehensions to start with pure aliases
+
+    e.g.
+    ```scala
+    for
+      a = 1
+      b <- Some(2)
+      c <- doSth(a)
+    yield b + c
+    ```
+2. Simpler conditional desugaring of pure aliases. i.e. whenever a series of pure aliases is not immediately followed by an `if`, use a simpler way of desugaring.
+
+    e.g. 
+    ```scala
+    for
+      a <- doSth(arg)
+      b = a
+    yield a + b
+    ```
+
+    will be desugared to
+
+    ```scala
+    doSth(arg).map { a =>
+      val b = a
+      a + b
+    }
+    ```
+
+    but
+
+    ```scala
+    for
+      a <- doSth(arg)
+      b = a
+      if b > 1
+    yield a + b
+    ```
+
+    will be desugared to
+
+    ```scala
+    doSth(arg).map { a =>
+      val b = a
+      (a, b)
+    }.withFilter { case (a, b) =>
+      b > 1
+    }.map { case (a, b) =>
+      a + b
+    }
+    ```
+
+3. Avoiding redundant `map` calls if the yielded value is the same as the last bound value.
+
+    e.g.
+    ```scala
+    for
+      a <- List(1, 2, 3)
+    yield a
+    ```
+
+    will just be desugared to
+
+    ```scala
+    List(1, 2, 3)
+    ```
+
+### Detailed description
+
+#### Ad 1. Allow `for` comprehensions to start with pure aliases
+
+Allowing `for` comprehensions to start with pure aliases is a straightforward change.
+
+The Enumerators syntax will be changed from:
+
+```
+Enumerators ::= Generator {semi Enumerator | Guard}
+```
+
+to
+
+```
+Enumerators ::= {Pattern1 `=' Expr semi} Generator {semi Enumerator | Guard}
+```
+
+Which will allow adding 0 or more aliases before the first generator.
+
+When desugaring is concerned, a for comprehension starting with pure aliases will generate a block with those aliases as `val` declarations and the rest of the desugared `for` as an expression. Unless the aliases are followed by a guard, then the desugaring should result in an error.
+
+New desugaring rule will be added:
+
+```scala
+For any N:
+  for (P_1 = E_1; ... P_N = E_N; ...)
+    ==>
+  {
+    val x_2 @ P_2 = E_2
+    ...
+    val x_N @ P_N = E_N
+    for (...)
+  }
+```
+
+e.g.
+
+```scala
+for
+  a = 1
+  b <- Some(2)
+  c <- doSth(a)
+yield b + c
+```
+
+will desugar to
+
+```scala
+{
+  val a = 1
+  for
+    b <- Some(2)
+    c <- doSth(a)
+  yield b + c
+}
+```
+
+#### Ad 2. Simpler conditional desugaring of pure aliases. i.e. whenever a series of pure aliases is not immediately followed by an `if`, use a simpler way of desugaring.
+
+Currently, for consistency, all pure aliases are desugared as if they are followed by an `if` condition. Which makes the desugaring more complicated than expected.
+
+e.g.
+
+The following code:
+
+```scala
+for
+  a <- doSth(arg)
+  b = a
+yield a + b
+```
+
+will be desugared to:
+
+```scala
+doSth(arg).map { a =>
+  val b = a
+  (a, b)
+}.map { case (a, b) =>
+  a + b
+}
+```
+
+The proposed change is to introduce a simpler desugaring for common cases, when aliases aren't followed by a guard, and keep the old desugaring method for the other cases.
+
+A new desugaring rules will be introduced for simple desugaring.
+
+```scala
+For any N:
+  for (P <- G; P_1 = E_1; ... P_N = E_N; ...)
+    ==>
+  G.flatMap (P => for (P_1 = E_1; ... P_N = E_N; ...)) 
+
+And:
+
+  for () yield E  ==>  E
+
+(Where empty for-comprehensions are excluded by the parser)
+```
+
+It delegares desugaring aliases to the newly introduced rule from the previous impreovement. i.e.
+
+```scala
+For any N:
+  for (P_1 = E_1; ... P_N = E_N; ...)
+    ==>
+  {
+    val x_2 @ P_2 = E_2
+    ...
+    val x_N @ P_N = E_N
+    for (...)
+  }
+```
+
+One other rule also has to be changed, so that the current desugaring method, of passing all the aliases in a tuple with the result, will only be used when desugaring a generator, followed by some aliases, followed by a guard.
+
+```scala
+For any N:
+  for (P <- G; P_1 = E_1; ... P_N = E_N; if E; ...)
+    ==>
+  for (TupleN(P, P_1, ... P_N) <-
+    for (x @ P <- G) yield {
+      val x_1 @ P_1 = E_2
+      ...
+      val x_N @ P_N = E_N
+      TupleN(x, x_1, ..., x_N)
+    }; if E; ...)
+```
+
+This changes will make the desugaring work in the following way:
+
+```scala
+for
+  a <- doSth(arg)
+  b = a
+yield a + b
+```
+
+will be desugared to
+
+```scala
+doSth(arg).map { a =>
+  val b = a
+  a + b
+}
+```
+
+but
+
+```scala
+for
+  a <- doSth(arg)
+  b = a
+  if b > 1
+yield a + b
+```
+
+will be desugared to
+
+```scala
+doSth(arg).map { a =>
+  val b = a
+  (a, b)
+}.withFilter { case (a, b) =>
+  b > 1
+}.map { case (a, b) =>
+  a + b
+}
+```
+
+#### Ad 3. Avoiding redundant `map` calls if the yielded value is the same as the last bound value.
+
+This change is strictly an optimization. This allows for the compiler to get rid of the final `map` call, if the yielded value is the same as the last bound pattern. The pattern can be either a single variable binding or a tuple.
+
+One desugaring rule has to be modified for this purpose.
+
+```scala
+  for (P <- G) yield P  ==>  G
+If P is a variable or a tuple of variables and G is not a withFilter.
+
+  for (P <- G) yield E  ==>  G.map (P => E)
+Otherwise
+```
+
+e.g.
+```scala
+for
+  a <- List(1, 2, 3)
+yield a
+```
+
+will just be desugared to
+
+```scala
+List(1, 2, 3)
+```
+
+### Compatibility
+
+This change may change the semantics of some programs. It may remove some `map` calls in the desugared code, which may change the program semantics (if the `map` implementation was side-effecting).
+
+For example the following code will now have only one `map` call, instead of two:
+```scala
+for
+  a <- doSth(arg)
+  b = a
+yield a + b
+```
+
+### Other concerns
+
+As far as I know, there are no widely used Scala 3 libraries that depend on the desugaring specification of `for`-comprehensions.
+
+## Links
+
+1. Scala contributors discussion thread (pre-SIP): https://contributors.scala-lang.org/t/pre-sip-improve-for-comprehensions-functionality/3509/51
+2. Github issue discussion about for desugaring: https://github.com/lampepfl/dotty/issues/2573
+3. Scala 2 implementation of some of the improvements: https://github.com/oleg-py/better-monadic-for
+4. Implementation of one of the simplifications: https://github.com/lampepfl/dotty/pull/16703
+5. Draft implementation branch: https://github.com/dotty-staging/dotty/tree/improved-fors