Transcendence

Eval is such a powerful function I know I will not think of all the cool things it can do.

the basic function is like this

(eval expression)

it has four basic modes (two that are trivial)

(eval number) returns that number
(eval function) returns that function

the two interesting cases are

(eval list) and (eval string)

if the expression is a list it will run that list as a called function

for example (eval (list "cat" 1 1)) will give you 11

so as you can see you can build your own code with this and run it

if the expression is a string it will treat that string as the name of the variable. (sorry it is hard to explain just think of this case as the opposite of set)

for example

(setq bob 11)
(setq myVar "bob")
(eval myVar)

the eval will return 11

beyond confusing people who look at your code what use is this?

well other than the storing code for later use you can store something that could be a global variable or a function. So say you have some data in a system some need a function called to find the data but some can just look at a global variable. With one simple line of code you can do both.

With this and apply you can have a class of things (like an item) where some have one function and others have different functions.

As I said earlier this is so powerful I know I am only scratching the surface if anyone else wants to add something please speak up or if you don't understand something please tell me so I can improve on this.

Interesting, I know I can use this.

If I have a function:
<Globals>
(block nil
(setq myFunction (lambda nil

(block (return)
(setq return (add 2 2))
)
))
)
</Globals>

and sometime I call eval like this:
(eval "myFunction")
I get 4 back?

What about if the function expects data? Is that what the list is used for?

(setq myFunction (lambda (a b)
(block (return)
(setq return (add a b))
)
))

and sometime I call eval like this:
(eval (list "myFunction" 2 2))

I get 4 back again?

and sometime I call eval like this:
(eval "myFunction")
I get 4 back?

nope you get the function back

it returns whatever is in the myFunction variable due to it being the string case

if you wanted the function run you would (eval (list "myFunction")) or using apply (apply myFunction nil) depending if you had the function itself (like you created it with lambda and passed it) or if you just had the name

(setq myFunction (lambda (a b)
(block (return)
(setq return (add a b))
)
))
and sometime I call eval like this:
(eval (list "myFunction" 2 2))

I get 4 back again?

yep you get the 4 back but remember that is due to it being a list and with lists it runs it as code.