Not to long ago, I sent an email to our documentation team because I sometimes like to throw them "grammar question of the day" emails. My last one was about the usage of the comma to delimit a person being addressed, exactly as defined in this example:
Let's eat, Grandma!
In spoken word, there is no comma there. People don't say, "let's eat <pause> Grandma," so why put the comma there? This is the response I got from a well respected technical writer and novelist:
I’m not an expert, but writing conventions do not mimic the cadence of spoken language. Instead they help readers decode words and phrases. This is an excellent example. In print, the comma helps the reader visually parse what was said, while distinguishing to whom it was said. In a novel, this occurs in almost every paragraph. Quotation marks and commas are used as delimiters so that the reader can keep track of who is speaking and listening.
So technically it's not incorrect to omit the comma, though it does define a cadence and provide clarity.
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