Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Initial proposal for language feature Continue For id and Exit For id #423

Open
wants to merge 2 commits into
base: main
Choose a base branch
from
Open

Initial proposal for language feature Continue For id and Exit For id #423

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
de165d8
Initial proposal for language feature `Continue For id` and `Exit For…
AdamSpeight2008 Apr 30, 2019
cfb9dae
Update proposal-Continue-Exit-For-With-Loop-Identifier.md
AdamSpeight2008 Jun 10, 2019
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
247 changes: 247 additions & 0 deletions proposals/proposal-Continue-Exit-For-With-Loop-Identifier.md
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,247 @@
# Continue Exit For Identifier

* [x] [Proposed](https://github.com/dotnet/vblang/issues/186)
* [x] Prototype: [Prototype/ContinueForIdentifier](https://github.com/AdamSpeight2008/roslyn-AdamSpeight2008/tree/Prototpye/ContinueForID)
* [ ] Implementation: [Not Started](https://github.com/dotnet/roslyn/BRANCH_NAME)
* [ ] Specification: [Not Started](pr/1)

## Summary
[summary]: #summary

Extend the capibility of `Contine For` and `Exit For` to enable continuing or exitting a loop nested higher.
Eg. `Continue For x` and `Exit For x`

-------

## Motivation
[motivation]: #motivation

* Why are we doing this?
* Seems like natural extension to the language, given that the `Next` can also have an identifer. eg `Next x`
* It can be difficult to implement workarounds, or require used different loop constructs at different nestings.
* Often implemented via `label`s and `GoTo`s. eg `GoTo start_of_loop_x`.
* What use cases does it support?
* This proposal is tightly focused on two cases;
* `Continue For loop_identifier`
* `Exit For loop_identifier`
* It can difficult to implement, resorting to using `label` and `GoTo` eg `GoTo start_of_loop_x`
* What is the expected outcome?
* Without an identifier eg `Continue For` or `Exit For` will perform exactly as it does now.
* With an identifier
* `Continue For x` will continue with the next iterator of the loop named with the identifier `x`.
* `Exit For x` will exit the loop named with the identifier `x`. Resuming execution at the next statement after it.

------

## Detailed design
[design]: #detailed-design

### Syntax Node Changes
An additional optional child node `<child name="ControlVariable" optional="true" kind="@ExpressionSyntax" />` will added to
* `ExitStatementSyntax`
* `ContinueStatementSyntax`
This additional node is used to identify which loop identifier to the statement refers to.
If the identifier is absent, the statement refers to current loop.

<details><summary>ExitStatementSyntax change</summary><p>

```xml
<!--****************
- Exit
******************-->
<node-structure name="ExitStatementSyntax" parent="ExecutableStatementSyntax">
<description>An exit statement. The kind of block being exited can be found by examining the Kind.</description>
<lm-equiv name="ExitNode"></lm-equiv>
<native-equiv name=""></native-equiv>
<spec-section>10.11</spec-section>
<grammar>ExitStatement</grammar>
<node-kind name="ExitDoStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitDo" />
</node-kind>
<node-kind name="ExitForStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitFor" />
</node-kind>
<node-kind name="ExitSubStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitSub" />
</node-kind>
<node-kind name="ExitFunctionStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitFunction" />
</node-kind>
<node-kind name="ExitOperatorStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitOperator" />
</node-kind>
<node-kind name="ExitPropertyStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitProperty" />
</node-kind>
<node-kind name="ExitTryStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitTry" />
</node-kind>
<node-kind name="ExitSelectStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitSelect" />
</node-kind>
<node-kind name="ExitWhileStatement">
<lm-equiv name="Exit" />
<native-equiv name="Statement.Opcodes.ExitWhile" />
</node-kind>
<child name="ExitKeyword" kind="ExitKeyword">
<description>The "Exit" keyword.</description>
<lm-equiv name="ExitKeyword"></lm-equiv>
<native-equiv name=""></native-equiv>
</child>
<child name="BlockKeyword" >
<description>The keyword describing the block to exit.</description>
<lm-equiv name="Block"></lm-equiv>
<native-equiv name=""></native-equiv>
<kind name="DoKeyword" node-kind="ExitDoStatement"/>
<kind name="ForKeyword" node-kind="ExitForStatement"/>
<kind name="SubKeyword" node-kind="ExitSubStatement"/>
<kind name="FunctionKeyword" node-kind="ExitFunctionStatement"/>
<kind name="OperatorKeyword" node-kind="ExitOperatorStatement"/>
<kind name="PropertyKeyword" node-kind="ExitPropertyStatement"/>
<kind name="TryKeyword" node-kind="ExitTryStatement"/>
<kind name="SelectKeyword" node-kind="ExitSelectStatement"/>
<kind name="WhileKeyword" node-kind="ExitWhileStatement"/>
</child>
<child name="ControlVariable" optional="true" kind="@ExpressionSyntax" />
</node-structure>
```
</p></details>

<details><summary>ContinueStatementSyntax change</summary><p>

```xml
<!--****************
- Continue
******************-->
<node-structure name="ContinueStatementSyntax" parent="ExecutableStatementSyntax">
<description>Represents a "Continue (block)" statement. THe kind of block referenced can be determined by examining the Kind.</description>
<lm-equiv name="ContinueNode"></lm-equiv>
<native-equiv name="Statement"></native-equiv>
<spec-section>10.11</spec-section>
<grammar>ContinueStatement</grammar>
<!-- REVIEW: What kind if just "Continue" is typed? Should there be different kinds? -->
<node-kind name="ContinueWhileStatement">
<lm-equiv name="Continue" />
<native-equiv name="Statement.Opcodes.ContinueWhile" />
</node-kind>
<node-kind name="ContinueDoStatement">
<lm-equiv name="Continue" />
<native-equiv name="Statement.Opcodes.ContinueDo" />
</node-kind>
<node-kind name="ContinueForStatement">
<lm-equiv name="Continue" />
<native-equiv name="Statement.Opcodes.ContinueFor" />
</node-kind>
<child name="ContinueKeyword" kind="ContinueKeyword">
<description>The "Continue" keyword.</description>
<lm-equiv name="ContinueKeyword"></lm-equiv>
<native-equiv name=""></native-equiv>
</child>
<child name="BlockKeyword">
<description>The "Do", "For" or "While" keyword that identifies the kind of loop being continued.</description>
<lm-equiv name="Block"></lm-equiv>
<native-equiv name=""></native-equiv>
<kind name="DoKeyword" node-kind="ContinueDoStatement"/>
<kind name="ForKeyword" node-kind="ContinueForStatement"/>
<kind name="WhileKeyword" node-kind="ContinueWhileStatement"/>
</child>
<child name="ControlVariable" optional="true" kind="@ExpressionSyntax" />
</node-structure>
```
</p></details>

### Parsing Changes
During parsing of the `ContinueForStatement` and `ExitForStatement` an additional check is performed to see if there is an identifer present.

Parse the optional identifier only if the statements are of the correct kind.
`ContinueForStatement` and `ExitForStatement`

### Binding Changes
During binding of the `ContinueForStatement` and `ExitForStatment`, we must validate that the identifer is a loop identifer, of a current or higher nesting level.

### Lowering Changes
The proposal achieves this via modifying how the `For` and `For Each` lowering are done. By extending the labels using during the lowering, to now include the loop's identifier. eg `StartOfForLoop_OuterLoop:` and `ExitForLoop_InnerLoop:`
So when the `Exit` or `Continue` is lowered, the corrispondig `GoTo` is to the correct location (thus correct nested loop) in the lowered code. eg `GoTo StartOfForLoop_OuterLoop` and `GoTo ExitForLoop_InnerLoop`

----------

<details><summary>Example Code:</summary><p>

```vbnet
Imports System
Module M1
Sub Main()
dim continueLoop as Boolean = true
For i = 0 To 2
Console.WriteLine($"Loop i Block Start ({i})")
For j = 0 To 2
Console.WriteLine($"Loop j Block Start ({i})")
If continueLoop Then
Console.WriteLine("Continuing")
continueLoop = false
Continue For i
End If
Console.WriteLine("Exiting")
Exit For i
Console.WriteLine($"Loop j Block End ({i})")
Next j
Console.WriteLine("After Loop j")
Console.WriteLine($"Loop i Block End ({i})")
Next
Console.WriteLine("After Loop i")
End Sub
End Module
```

<details><summary>Expected Output</summary><p>

```
Loop i Block Start (0)
Loop j Block Start (0)
Continuing
Loop i Block Start (1)
Loop j Block Start (0)
Exiting
After Loop i
```

</p></details>

</p></details>


-----

## Drawbacks
[drawbacks]: #drawbacks

Why should we *not* do this?

------

## Alternatives
[alternatives]: #alternatives

What other designs have been considered? What is the impact of not doing this?

----

## Unresolved questions
[unresolved]: #unresolved-questions
What parts of the design are still TBD?

* **TIED TO VB15** need to be updated, if accepted.
* Checking of the validity of the identifer used in the `Continue` and `Exit`
* Additional error messages
* Additional use cases. *Not covered in this proposal, has it require provide a name for the loop constuct.*
* `Do ... Loop`
* `While .. End While`