Details of GNU Common Lisp's (type-of)

Question

If at the REPL I enter:

(type-of (make-array 5))

then I get the response:

(SIMPLE-VECTOR 5)

Fair enough. So if at the REPL I enter:

(type-of (make-array (list 5 3 2)))

then I get the response:

(SIMPLE-ARRAY T (5 3 2))

I have two questions.

What is the T telling me here? If it had been NIL instead, what would that have told me?
Where could I have found this answer on my own? I failed to find the answer in (for example) the Lisp HyperSpec.

GNU Common Lisp (short GCL) and GNU CLISP are two different implementations of Common Lisp. You don't need to tag general questions with all kinds of implementation names. — Rainer Joswig
– Rainer Joswig, Commented Jan 3, 2012 at 19:20

Rainer Joswig · Accepted Answer · 2011-12-01 13:29:40Z

(SIMPLE-ARRAY T (5 3 2)) is a simple array of three dimensions. T says that it is a general array which can contain any element type. T is the most general type.

The hyperspec documents the type SIMPLE-ARRAY here:

http://www.lispworks.com/documentation/HyperSpec/Body/t_smp_ar.htm

Ooo. Thanks! Your link was tough sledding for me, but ultimately useful! I figured out, for example, that (type-of (make-array 5 :element-type (integer 0 255))) yields (SIMPLE-ARRAY (UNSIGNED-BYTE 8) (5)).

Vatine · Accepted Answer · 2011-12-01 15:38:47Z

1) If the T had been NIL, you would have a three-dimensional array, specialised in not storing data (no element has the type NIL; I believe all types are a super-type of NIL).

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