How to cast a character to int in Clojure?

What you are looking for is a character literal \a. A character literal is denoted by a single character, or 16-bit unicode code point, prefixed by the \ reader macro.

(int \a) ;; => 97

(int \0) ;; => 48

(int \u0030) ;; => 48

With (int a), a is a symbol. As such, the runtime tried and failed to resolve what that symbol was bound to.

With (int 'a), a is also a symbol, but, because you declared it a symbol with the single quote ('), the runtime took it literally and tried and faild to cast the clojure.lang.Symbol to a java.lang.Character.

With (rot13 ''a'), 'a' declares a' as a symbol. But, the extra ' prefixing it makes the runtime treat the expression that declared the a' literally. 'a' expands to (quote a'), so the "literal literal", ''a', expands to the list (quote a').

''a' ;; => (quote a')

(second ''a') ;; => a'

With (rot13 "a"), a is a string. Strings cannot be cast to characters, but they can be treated as collections of characters. So, (rot13 (first "a")) would work as intended.

Clojure will convert characters to int automatically if you ask nicely. It will convert it using ascii equivalence (well unicode in fact).

(int \a);;=>97
(int \b);;=>98
(int \c);;=>99

However, if you want to convert a Clojure char that is also a number to an int or long :

(Character/digit \1 10) ;; => 1
(Character/digit \2 10) ;; => 2

A non-digit char will be converted to -1 :

(Character/digit \a 10) ;;=> -1

Which is ok in this case, since \-1 is not a character.

(Character/digit \-1 10);;=>java.lang.RuntimeException: Unsupported character: \-1

It could be convenient also to note that - would convert to -1, although I wound not rely on it too much :

(Character/digit \- 10);;=>-1

The second parameter is obviously the base. Ex :

(Character/digit \A 16);;=>10