Unfortunately, J's grammar isn't the cleanest. It uses juxtaposition to mean about a dozen different things depending on whether the tokens are data, first-orde

Unfortunately, J's grammar isn't the cleanest. It uses juxtaposition to mean about a dozen different things depending on whether the tokens are data, first-order functions, or second-order functions and how many are juxtaposed. A variable could stand for any of these. If there are variables in the expression, you have to know which variables stand for which kinds of objects in order to parse the expression.