a lot of the time there are patterns and idioms that occur very frequently in the language, and have evolved a set of standard short names, e.g. "xs" for a list

a lot of the time there are patterns and idioms that occur very frequently in the language, and have evolved a set of standard short names, e.g. "xs" for a list and then "x" for the element in an iteration, or "acc" for an accumulator parameter in a recursive function.