let vs def in clojure

lisp clojure let function
Jason Baker picture Jason Baker · Mar 8, 2009 · Viewed 10.5k times · Source

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?

Answer

Rayne picture Rayne · Mar 8, 2009

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.

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