In a large pot of boiling salted water, cook pasta 3 minutes less than al dente. Add snap peas; cook 2 minutes, add peas, and cook 1 minute more. Reserve 1/2 cup pasta water; drain pasta and vegetables, and return to pot.
Toss pasta and vegetables with butter, tarragon, and ricotta, adding a little reserved pasta water as needed to create a thin sauce that coats pasta; season with salt and pepper.
Divide pasta among four serving bowls, and garnish with tarragon, if desired. Serve immediately.
To trim snap peas, cut off stem end with a knife. Or, if the string is especially tough, snap off stem end with fingers, and pull string down the pod.