what is [#^String str] in Clojure function type hint

Question

I saw a Clojure function like

(defn strFun
[#^String str]
(...))

I guess the #^String is type hint but what is the #and ^ before the String?

Answer 1

It's the old format for specifying simple metadata tags before it was just ^TagName. You will see it in code written during this transition from time to time, though there is no need to use it.

user> #^String ["hello"]                
["hello"]                               
user> (meta #^String ["hello"])         
{:tag java.lang.String}                 

is the same as not using the #

user> (meta ^String ["hello"])          
{:tag java.lang.String}                 
user> 

PS: in this example I tagged a vector with the tag java.lang.String. This point to note is that the symbol you use as a tag will be resolved and the value that symbol resolves to will be used as the tag. So you can't use an undefined symbol.