Question

I discovered that special forms can not be passed as arguments or saved in variables, both in Clojure:

user=> (defn my-func 
         [op] 
         (op 1 2 3))
#'user/my-func
user=> (my-func +)
6
user=> (my-func if)
java.lang.Exception: Unable to resolve symbol: if in this context (NO_SOURCE_FILE:5)
user=> (def my-if if)
java.lang.Exception: Unable to resolve symbol: if in this context (NO_SOURCE_FILE:9)

and in Racket/Scheme:

> (define (my-func op)
    (op 1 2 3))
> (my-func +)
6
> (my-func if)
  if: bad syntax in: if
> (define my-if if)
  *if: bad syntax in: if

That’s cool, I’m fine with that, I know that I can just write a function to wrap a special form and then pass that function.

But I’d like to know why Lisps have this restriction, and what negative consequences allowing this would have. Are they any major dialects that do allow that?

Answer 1

It makes evaluation more complicated and compilation hard.

If you have a form (a b c), then you need to resolve at runtime the value of a and then apply it somehow to the forms b and c.

The simpler model of a few limited number of special forms and otherwise strict evaluation is then gone.

See also: FEXPR