Proposal: Allow Single Dollar Sign as Valid Identified (#354)

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)
+* Review manager: TBD
+* Status: **Awaiting review**
+
+## Introduction
+
+Currently, the Swift compiler does not throw any errors when `$`
+character (U+0024) is used as an identifier by itself. For example:
+
+```swift
+let $ = 10
+// OR
+let $ : (Int) -> (Int) = { $0 * $0 }
+// OR
+class $ {}
+```
+
+Swift language should continue to support the use of `$` as a valid
+identifier in future versions of Swift (>= 3.1).
+
+## Motivation
+
+Currently a lot of projects depend on the following library
+[Dollar](https://github.com/ankurp/Dollar), which uses `$` character (U+0024)
+as a namespace because it was a valid identifier since Swift was released.
+The core team has decided to remove it as a valid character by merging this
+[Pull Request](https://github.com/apple/swift/pull/3901)
+
+The reason behind the removal of `$` character as a valid identifier is
+to make the behavior consistent as `$` 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 $a = 5
+```
+
+Also they wish to reserve `$` for future debugging tools.
+
+## Proposed solution
+
+Allow `$` character (U+0024) to be used as a valid identifier without use of
+any tick marks `` `$` ``.
+
+## Impact on existing code
+
+This proposal will preserve existing syntax of allowing `$` to be used as a
+valid identifier by itself, so there will be no impact on existing code. But
+if `$` (U+0024) character is removed from being used as a valid identifier
+then it will impact a lot of developers who use the
+[Dollar](https://github.com/ankurp/Dollar) library.
+
+## Alternatives considered
+
+The primarily alternative here is to allow for the breaking change and use 
+`` `$` `` as the identifier in the [Dollar](https://github.com/ankurp/Dollar)
+library.