If a string in lisp is a vector, why can't I access the first element using svref?

Question

So, I'm trying to learn Lisp, and I've come across a problem in the definition of what a String is.

I'm reading the ANSI Common Lisp by Paul Graham, and in this book it states that a String is a vector, or one-dimensional Array.

So, I create a String:

(defvar *my-string* "abc")

And then I can access the first value of my-string this way:

(aref *my-string* 0)

But if it is a vector, why can't I access that element this way:

(svref *my-string* 0)

I mean, when I create a vector this way:

(defvar my-vec (make-array 4 :initial-element 1))

I can access first element using svref:

(svref my-vec 0) ; returns 1

I forgot to add error when I try svref on String:

"The value "abc" is not of type (SIMPLE-ARRAY T (*))."

Answer 1

String is a vector, but it isn't a simple-vector. svref takes a simple-vector as first argument.

You can check it by calling:

(vector-p *my-string*)

which returns true

Unlike:

(simple-vector-p *my-string*)

which returns false.

Notice that (simple-vector-p my-vec) will return true as well, which confirms that make-array creates a simple-vector.

Answer 2

soulcheck's answer is absolutely right, but it's worth the time to become comfortable with the HyperSpec. For instance, if you start at the page for svref, there's a note at the bottom:

Notes:

svref is identical to aref except that it requires its first argument to be a simple vector.

The glossary entry for simple vector (linked above) says:

simple vector n. a vector of type simple-vector, sometimes called a "simple general vector." Not all vectors that are simple are simple vectors—only those that have element type t.

15.2 The Arrays Dictionary is also helpful here, as is 15. Arrays as a whole.