Evaluate a parse tree

eval x     eval[x]

Where x is a parse tree, returns the result of evaluating it.

The eval function is the complement of parse and can be used to evaluate the parse trees it returns. (Also parse trees constructed explicitly.)

q)parse "2+3"
+
2
3
q)eval parse "2+3"
5
q)eval (+;2;3)      / constructed explicitly
5