Have you seen the message
error: repl.swift:2:31: error: '<type>' is only available on OS X <version> or newer? In current releases of Xcode, the Swift REPL uses a hard-coded 10.9 target instead of the current OS.
The solution is fairly easy:
xcrun swift -target x86_64-apple-macosx10.12
Substitute whatever OS version or platform you need.
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