The 5E Druid spell Thorn Whip has a hit effect that pulls the target (Large or smaller) 10 feet, in addition to dealing piercing damage. I imagine this hit effect as the whip wrapping itself around the target and pulling.
Can I choose to not pull the target on a hit? I am imagining just doing a lashing strike, just dealing the piercing damage.
Yes, it is a choice. Or more specifically, you must pull the creature, but you can pull him 0 feet.
Thorn Whip
You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range.
Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.
The spell does exactly what it says. Many spells and abilities give you the ability to choose, by including the appropriate wording. This one specifically allows you to choose the length you pull the creature (0-10 feet).
Yes, you can choose not to pull the target. Or, to be more precise, you can choose to pull the target 0 feet.
If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.
The "up to" is quite important - it means that you can pull the creature a maximum of 10 feet, not an absolute value of 10 feet.
