On 31 May 2010, at 11:08, alux wrote:
Hello Konrad, many thanks, that clarifies it!
So, in "A special form is always built in, and cannot be implemented
in the language itself." the first part is true, the second is not.
Like most lisps have a bilt in list-reverse for speed reasons.
In that case, this build in thing is a special form that can also be
implemented in the language (only slow).
Right?
Right.
"A special form is named by a symbol whose interpretation
cannot be overridden in any way."
This explains the problem from another thread (that made me ask this
question in the first place):
user=> (def do println)
#'user/do
; this uses the special form
user=> (do "huhu" "plop")
"plop"
; while this uses println
user=> ((var do) "huhu" "hello")
huhu hello
nil
Yes, that illustrates the handling of special forms nicely. Note that
it is somewhat specific to Clojure. Other Lisps handle the mapping
from symbols to functions/macros/special forms differently.
One more variant is helpful:
user=> (user/do "huhu" "hello")
huhu hello
nil
This illustrates that special forms need to be identified by an
unqualified symbol in the operator position.
Another detail that is good to know is that syntax-quote (`) knows
about special forms and treats their symbols specially even if they
are not in the operator position:
user=> (declare do foo)
#'user/foo
user=> `do
do
user=> `foo
user/foo
Konrad.
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