Support the ternary operator in PowerShell language

1-Draft/RFCxxxx-Ternary-Operator.md

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+---
+RFC: RFCxxxx
+Author: Dongbo Wang
+Status: Draft
+SupercededBy: N/A
+Version: 0.1
+Area: Language
+Comments Due: August 31st, 2019
+Plan to implement: Yes, PS7
+---
+
+# Add Ternary Operator to PowerShell Language
+
+The ternary operator is one of the highly demanded enhancement to PowerShell language.
+It received 46 up-votes in the issue [Suggestion: implement ternary conditionals](https://github.com/PowerShell/PowerShell/issues/3239) as of the writing of this RFC.
+The [prototype draft pull request](https://github.com/PowerShell/PowerShell/pull/10161) for this feature has also received a lot of attention and discussions.
+
+The PowerShell Committee has approved to add the ternary operator to the language,
+and this RFC is mainly to capture the design points and implementation details.
+
+## Motivation
+
+> As a PowerShell user, I can use ternary operator as a replacement of if-else statement for simple conditional cases.
+
+## Specification
+
+We will follow the C# ternary operator syntax:
+
+```none
+<condition-expression> ? <if-true-expression> : <if-false-expression>
+```
+
+Two basic design points:
+
+- Ternary operator produces an expression, just like the binary and unary operators
+- Ternary operator has lower precedence order than the binary operator
+
+The ternary operator behaves like the simplified if-else statement.
+The `condition-expression` will always be evaluated,
+and its result will be converted to boolean to determine which branch will be evaluated next:
+
+- `if-true-expression` will execute if the condition's result is evaluated as `true`
+- `if-false-expression` will execute if the condition's result is evaluated as `false`
+
+Some usage examples (see more examples [in the tests](https://github.com/PowerShell/PowerShell/pull/10367/files#diff-b63d92046948a571e210b3abb5a7e685)):
+
+```powershell
+...
+$psExec = $IsCoreCLR ? 'pwsh' : 'powershell'
+...
+
+...
+$argument = $env:CI -eq 'true' ? '-Tag CI' : '-Tag Daily'
+...
+
+...
+$message = (Test-Path $path) ? "Path exists" : "Path not found"
+...
+
+...
+return $blackList -contains $target ? (Block-Target $target) : (Register-Target $target)
+...
+```
+
+#### Parser Updates
+
+A new AST type `TernaryExpressionAst` will be introduced to represent a ternary operator expression.
+The public API surface of this type is as follows:
+
+```c#
+public class TernaryExpressionAst : ExpressionAst
+{
+    public TernaryExpressionAst(IScriptExtent extent, ExpressionAst condition, ExpressionAst ifTrue, ExpressionAst ifFalse);
+    public ExpressionAst Condition { get; }
+    public ExpressionAst IfTrue { get; }
+    public ExpressionAst IfFalse { get; }
+
+    public Ast Copy();
+}
+```
+
+The challenge in the parser update is around how to deal with number constant expressions more naturally with the ternary operators.
+When digits are mixed with `?` or `:`, such as `?123`, `:456`, `123:` and `?123:456`, they are recognized as generic tokens today,
+because it's totally legit to have a function or script with those names. However, when it comes to the ternary expression,
+this means confusing errors would occur for scripts like `return ${succeed}?0:1` since `?0:1` would be treated as a generic token.
+Therefore, spaces around `?` and `:` would be required to use number constant expressions with the ternary operator.
+That would certainly be counter-intuitive and should be addressed in the parser update.
+
+The approach I took in the prototype is to make `?` and `:` conditionally force to start a new token when scanning for numbers.
+When it's known for sure that we are expecting an expression,
+allowing a generic token like `123?` is not useful and bound to result in parsing errors.
+In those cases, we will force to start a new token upon seeing characters `?` and `:` when scanning for a number,
+so that that number constant expressions can work with ternary operators more intuitively.
+
+Note that, the handling of `?` and `:` doesn't change how those two characters can be used in variable expressions,
+and also doesn't affect function and commands with those two characters in names.
+
+#### AST Visitors
+
+Given the fact that we are adding a new AST type, necessary changes are required to `ICustomAstVisitor2`, `AstVisitor2`,
+and all other visitor types that implemnent those two.
+Obvious examples are the `VariableAnalysis` and `Compiler` as you have to update them properly to correctly enable the new language element.
+The not-so-obvious ones include `ConstantValueVisitor`, `SafeValueVisitor`, `TypeInferenceVisitor` and more.
+Take the `TypeInferenceVisitor` as an instance, failing to update it may cause tab completion issues that are hard to catch.
+
+It's worth to discuss the `InternalVisit` implemention of the new AST type a bit.
+Many `Find*Visitor` types depend on this method to search through an AST tree,
+and almost all of them derive from `AstVisitor` instead of `AstVisitor2` because
+currently the AST types covered in `AstVisitor2` all have their own special scopes,
+such as the `TypeDefinitionAst` and `DynamicKeywordStatement`.
+In order to avoid making changes to all those existing `Find*Visitor` types,
+the `InternalVisit` method of `TernaryExpressionAst` is implemented as follows to accept an `AstVisitor` visitor for its AST members.
+In this way, those existing `Find*Visitor` types can work with this new AST type to search through its members.
+
+```c#
+// This pattern appears in several other ASTs.
+internal override AstVisitAction InternalVisit(AstVisitor visitor)
+{
+    var action = AstVisitAction.Continue;
+    if (visitor is AstVisitor2 visitor2)
+    {
+        action = visitor2.VisitTernaryExpression(this);
+        if (action == AstVisitAction.SkipChildren)
+            return visitor.CheckForPostAction(this, AstVisitAction.Continue);
+    }
+
+    if (action == AstVisitAction.Continue)
+        action = Condition.InternalVisit(visitor);
+
+    if (action == AstVisitAction.Continue)
+        action = IfTrue.InternalVisit(visitor);
+
+    if (action == AstVisitAction.Continue)
+        action = IfFalse.InternalVisit(visitor);
+
+    return visitor.CheckForPostAction(this, action);
+}
+```
+
+### Alternate Proposals and Considerations
+
+Different syntax proposals were brought up in the discussions:
+
+```none
+## Use '-then -else' instead of '? :'
+<condition-expression> -then <then-expression> -else <else-expression>
+```
+```none
+## Use syntax similar to '-replace' and '-f'
+<condition-expression> -ternary <then-expression>, <else-expression>
+```
+
+After a discussion at length, the PowerShell committee agrees to align with the C# syntax given
+the expectation that most usage of this new language element will be from C# developers.