Proposal: Allow Single Dollar Sign as Valid Identified

Author: ankurp

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

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+# Allow Single Dollar Sign as Valid Identified
+
+* Proposal: [SE-0000]()
+* Author: [Ankur Patel](https://github.com/ankurp)
+* Status: **Awaiting review**
+* Review manager: TBD
+
+## Introduction
+
+Currently, the compiler does not generate an error when the
+identifier `$` is used by itself. Swift should continue supporting
+use of `$` as an identifier but when used with non-numeric name
+consider it an error. 
+
+## Motivation
+
+Currently a lot of projects depend on the following library 
+[Dollar](https://github.com/ankurp/Dollar) which uses `$` as 
+a namespace since it was valid since when 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 problem is that when used with a non-numeric name (e.g. $a), 
+the compiler raises the correct error:
+
+```swift
+ERROR at line 1, col 5: expected numeric value following '$'
+var $a = 5
+```
+
+This may seem like inconsistent behavior to some causing the 
+removing of `$` as a valid identifier in the language. 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)