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add reflection to readme

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Pekka Laiho 2020-12-05 09:02:53 +07:00
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@ -379,6 +379,34 @@ PI | PHP constant `M_PI`
\_\_DIR\_\_ | Directory of a file being evaluated using the special form `load`. Otherwise null.
\_\_FILE\_\_ | Filename of a file being evaluated using the special form `load`. Otherwise null.
## Reflection
You can use the `meta` function to retrieve the arguments, body or full code of user-defined functions:
```
> (def add (fn (a b) (+ a b)))
<function>
> (meta add "args")
(a b)
> (meta add "body")
(+ a b)
> (meta add "code")
(fn (a b) (+ a b))
```
This allows for some fun tricks. For example, we can retrieve the body of a function and evaluate it as part of another function:
```
> (def addOne (fn (n) (+ n 1)))
<function>
> (def addTwo (fn (n) (+ n 2)))
<function>
> (def addBoth (fn (n) (+ (eval (meta addOne "body")) (eval (meta addTwo "body")))))
<function>
> (addBoth 1)
5
```
## Extending
The project is easy to extend because it is trivial to add new functions whether the implementation is defined on the PHP or Lisp side.