Proposal: Allow Single Dollar Sign as Valid Identified

Author: ankurp

proposals/0000-allow-single-dollar-sign-as-valid-identifier.md

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+# Allow Single Dollar Sign as Valid Identified
+
+* Proposal: [SE-NNNN](NNNN-allow-single-dollar-sign-as-valid-identifier)
+* Author: [Ankur Patel](https://github.com/ankurp)
+* Status: **Awaiting review**
+* Review manager: TBD
+
+## Introduction
+
+Currently, the compiler does not generate an error when `$` character is used
+as an identifier by itself. Swift should continue supporting use of `$` as a
+valid identifier in Swift 3 and later versions of Swift.
+
+## Motivation
+
+Currently a lot of projects depend on the following library 
+[Dollar](https://github.com/ankurp/Dollar) which uses `$` as a namespace because
+it was a valid identifier since Swift was released. Now the core team is
+deciding to remove it as a valid character according to this bug ticket 
+[SR-1661](https://bugs.swift.org/browse/SR-1661)
+
+The reason why the core team wishes to remove `$` as a valid identifier is
+to make the behavior consistent as `$` when suffixed with a valid identifier,
+(i.e. \$[a-zA-Z_....]), will raise an error:
+
+```swift
+ERROR at line 1, col 5: expected numeric value following '$'
+var $a = 5
+```
+
+Also `$` may be reserved for debugging tools in the future.
+
+## Proposed solution
+
+Allow `$` to be used a valid identifier and reserve `$$` (double dollar sign)
+for namespacing debugger tools in the future when they are introduced.
+
+## Impact on existing code
+
+This functionality is keeping the existing sytax in place so there will be no
+impact of existing code. But if the core team decides to remove the `$` as a
+valid identifier then it will impact a lot of developers code as it is being
+used indirectly by importing [Dollar](https://github.com/ankurp/Dollar) library.
+
+## Alternatives considered
+
+The primarily alternative here is to do nothing by closing this ticket 
+[SR-1661](https://bugs.swift.org/browse/SR-1661)