I want to make a local instance of a Java Scanner class in a clojure program. Why does this not work:
; gives me: count not supported on this type: Symbol
(let s (new Scanner "a b c"))
but it will let me create a global instance like this:
(def s (new Scanner "a b c"))
I was under the impression that the only difference was scope, but apparently not. What is the difference between let and def?
The problem is that your use of let is wrong.
let works like this:
(let [identifier (expr)])
So your example should be something like this:
(let [s (Scanner. "a b c")]
(exprs))
You can only use the lexical bindings made with let within the scope of let (the opening and closing parens). Let just creates a set of lexical bindings. I use def for making a global binding and lets for binding something I want only in the scope of the let as it keeps things clean. They both have their uses.
NOTE: (Class.) is the same as (new Class), it's just syntactic sugar.
LET is not "make a lexical binding in the current scope", but "make a new lexical scope with the following bindings".
(let [s (foo whatever)] ;; s is bound here ) ;; but not here
(def s (foo whatever)) ;; s is bound here