Why does single quote in Lisp always return upper case?

Question

I'd like to be able to set case from a single quote, but that does not seem possible.

(format nil "The value is: ~a" 'foo)
"The value is: FOO"

(format nil "The value is: ~a" 'FOO)
"The value is: FOO"

(format nil "The value is: ~a" "Foo")
"The value is: Foo"

Answer 1

Quoting

The quote has nothing to do with case. A quote prevents evaluation.

quoting a symbol:

CL-USER 1 > 'foo
FOO

quoting a list:

CL-USER 2 > '(1 2 3 foo)
(1 2 3 FOO)

You can put a quote in front of many things. For example in front of a string:

CL-USER 3 > '"a b c"
"a b c"

Since strings evaluate to themselves, quoting them or not makes no difference:

CL-USER 4 > "a b c"
"a b c"

Symbols are by default read as uppercase:

CL-USER 5 > 'FooBar
FOOBAR

CL-USER 6 > (symbol-name 'FooBar)
"FOOBAR"

But that has nothing to do with quoting and is a feature of the reader.

CL-USER 7 > (read-from-string "foo")
FOO
3

Downcase

If you want the string in lowercase, you need to convert the string to lowercase:

CL-USER 8 > (string-downcase (symbol-name 'FooBar))
"foobar"

Symbols with mixed case

But you can create symbols with lowercase names or mixed case. You need to escape them:

CL-USER 9 > '|This is a symbol With spaces and mixed case|
|This is a symbol With spaces and mixed case|

CL-USER 10 > 'F\o\oB\a\r
|FooBar|

Downcasing output using FORMAT

You can also tell FORMAT to print in lowercase:

CL-USER 11 > (format nil "The value is: ~(~a~)" 'foo) 
"The value is: foo"