Andy Smith writes: > I came across the following issue when doing problem > > user=> (let [f '(+ 1 1)] (apply (first f) (rest f))) > 1 > > I could use eval but eval is bad (apparently)... > > Investigating this further I can see that : > > user=> (let [f '(+ 1 1)] (type (first f))) > clojure.lang.Symbol > > wheras : > > user=> (type +) > clojure.core$_PLUS_ > > It would seem that I still need to call eval on the '+ at least, in order > to resolve it to a function, but eval is still bad (apparently). > > I could create a map of symbol to function and I can see that others have > done the same, but this seems a bit verbose because clojure will do that > for me with eval. If the map is the correct approach then why is it any > better than eval? > > What is the correct thing to do here? Im sure this is revealing some > fundamental misunderstanding that I have... :o/
You want to read about the connection between Vars, Namespaces and Symbols. Please don't see the following as a good summary for these topics, it just illustrates the concepts in the scope of your problem: A var, for example #'clojure.core/+, is a way to refer to a storage location. #'clojure.core/+ is a var which 'contains' a function which adds numbers. A Namespace (for example clojure.core) is just a collection of vars. A symbol is a form of identifier. '+ is a symbol. You start with the symbol +. The function `(ns-resolve ns sym)' will try to find a var in `ns' with the same name as `sym' and return it. In the end, you want to call the function behind the var, so you have to use `var-get' to get the value the var points to. -- Moritz Ulrich
pgphkZl7qIu1U.pgp
Description: PGP signature
