I've written an implementation of list comprehensions for Common
Lisp. My implementation also allows manipulation of hash tables and
arrays.

Documentation and source code can be downloaded at
<http://www.csd.uu.se/~svenolof/collect.tar.gz>.

  (collect (list) ((* x x))
    (in (x) '(1 2 3 4 5 6 7 8)))

Is there a good reason for preferring this over

  (collect list (* x x)
    (in (x) '(1 2 3 4 5 6 7 8)))

In other words, do the parentheses in

  <type-exp> ::= (nil) | (t) | (list) | (vector)

If we are only interested in working with lists and (one-dimensional)
arrays, the syntax you suggest is sufficient. However, if we want to
collect the result in a hash table, we want to say how the values with
a particular key are o be collected. For example:

(collect (hash-table (vector)) ((mod x 2) x)
  (in (x) '(1 2 3 4 5 6)))

Here, all values with the same key are collected in a vector. The
collect expression generates a sequence of key-value pairs (the key
given by (mod x 2)). Thus, the result of the collect expression is
(under clisp):

#S(HASH-TABLE EQL (0 . #(2 4 6)) (1 . #(1 3 5)))