Q: I’m puzzled by this sentence: “Uber’s surge pricing algorithm does not reduce output, it increases the supply of transportation providers.” Technically, it’s a run-on sentence and incorrect. But it feels so right. What are your thoughts?
A: It’s true that in general you shouldn’t use a comma alone—without a conjunction like “and” or “but”—to join two independent clauses (that is, clauses that could stand alone as separate sentences).
Supposedly, to use a comma instead of a semicolon creates a “spliced” or “run-on” sentence. Or so we’ve been taught.
But we think the example you sent is fine as it is. In our opinion, it’s not a “run-on” sentence.
This is a natural (and very common) way of writing everyday English. In more formal—perhaps legal or academic—writing, you might prefer a semicolon to a comma.
The Cambridge Grammar of the English Language has a discussion of “what prescriptivists call a ‘spliced’ or ‘run-on’ comma,” and it provides this example: “The locals prefer wine to beer, the village pub resembles a city wine bar.”
Such a sentence would be “widely regarded as infelicitous,” say the authors, Rodney Huddleston and Geoffrey K. Pullum.
But in certain cases, commas are accepted when used to join two independent clauses, Huddleston and Pullum write. They give these examples:
(1) “To keep a child of twelve or thirteen under the impression that nothing nasty ever happens is not merely dishonest, it is unwise.”
Here a negative clause is followed by a positive, the authors note. In such cases—especially where the negative clause has “not only,” “not simply, “not merely,” or “not just”—the positive clause often starts with “but.” The authors add that the “construction without but is also common, however, and readily allows the comma.”
(2) “Some players make good salaries, others play for the love of the game.”
Here, the Cambridge Grammar explains, “The comma is justified by the close parallelism between the clauses and their relative simplicity.”
The sentence you ask about—“Uber’s surge pricing algorithm does not reduce output, it increases the supply of transportation providers”—is like No, 1, with a negative clause followed by a positive. It also resembles No. 2 in that the two independent clauses are closely parallel.
This is why we don’t consider it a “run-on” sentence, and why we think the comma is fine.
In a footnote, the Cambridge Grammar mentions a third kind of sentence in which a comma is used to separate two independent clauses. The example given is “Order your furniture on Monday, take it home on Tuesday.”
Technically, the authors write, these are two separate imperative clauses. But the sentence “is interpreted as a conditional statement, ‘If you order your furniture on Monday you can take it home on Tuesday.’ ”
Using a semicolon instead of a comma (“Order your furniture on Monday; take it home on Tuesday”), the authors write, “would allow only the literal interpretation as a compound directive.”
Here’s something else to keep in mind. As the Cambridge Grammar points out, the great mass of published English that we read is edited according to “codified rules” of punctuation that are “set out in manuals specific to a particular publishing house or accepted more widely as authoritative guides.”
Despite this “codification,” the Cambridge Grammar says, “punctuation practice is by no means entirely uniform.” As we’ve written on our blog, punctuation also changes with the times.
from Grammarphobia http://www.grammarphobia.com/blog/2017/03/run-on.html