Swift snapshots for Ubuntu 18.04

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I installed swift for ubuntu 18.04 development release, going through this tutorial. When I enter swift command, REPL starts. I can define an integer using "let" command too. But when I want to define a new string, face this error:
"error: Couldn't lookup symbols:
Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String؛

swift_error
swift_error734×350 33 KB

I'm new to swift and can't understand what does that error exactly mean. Can anyone help me?

thanks!