Working through FP for OO Programers by @marick

differences-between-a.clj

;solution presented
(def a
  (fn [type & args]
    (apply type args)))

;my original solution
(def a
  (fn [type & args]
    (eval (cons type args))))

;which I realised was the same as this
(def a
  (fn [& args]
    (eval args)))

;The solution presented in the book is definitely clearer as to what's going on.
;I'm wondering though if there's anything to be aware of about using eval like this?

In this particular case, it doesn't matter. The big difference between eval and apply is that eval can't "see" symbols bound in a let. Compare this:

user=> (let [args [1]]
         (apply inc args))
2
user=> (let [args [1]]
         (eval '(inc args)))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: args in this context, compiling:(NO_SOURCE_PATH:169) 

This doesn't become important until (I think) chapter 10.

Probably a clearer example of how eval is blind would just be this:

user=> (let [args [1]]
         (eval '(inc args)))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: args in this context, compiling:(NO_SOURCE_PATH:169) 

Urk. Pasted wrong thing:

user=> (let [x 1] (eval 'x))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: x in this context, compiling:(NO_SOURCE_PATH:171)