`Collect` AST node should accept a complex expression in order to cooperate with L_Array

[leungbk]

In the L_Array language, the abstract syntax includes (make-vector exp exp), where both arguments can be complex expressions. This would appear to include the case where the first exp happens to be a (read) invocation (or a Var bound to a read invocation).

When rewriting the expose_allocation pass for L_Array, the book suggests that we generate code for L_Array in a manner analogous to the case for L_Tup, which, among other things, generates a Collect node that accepts a plain integer (and not an Int AST node). In the case of L_Tup, the integer is always known at compile time, since in the compiler pass, we can simply count the entries passed into (vector entry-0 entry-1...), so it is fine to create a Collect AST node with that known length; however, for L_Array, if the specified length is a complex expression that cannot be known at compile time (such as a (read) invocation), then Collect in its present form does not seem appropriate. This is confusing to me, especially since it is stated in the text that AllocateArray takes a complex subexpression; possibly I am simply missing the right way to rewrite expose_allocation for L_Array.