Please let me know. &OPTIONAL parameters.

[だいとうりょう むがべ]

I'm Japanese newbie. Excuse me in halting English.
Please let me know.
from CLHS. &OPTIONAL parameters.
http://www.lispworks.com/documentation/HyperSpec/Body/03_daf.htm

((lambda (&optional (a 2 b) (c 3 d) &rest x) (list a b c d x)))
=> (2 NIL 3 NIL NIL)
a is 2.
b is 3.
other nil.OK

((lambda (&optional (a 2 b) (c 3 d) &rest x) (list a b c d x)) 6)
=> (6 T 3 NIL NIL)

T is the what?. From where?
"b" value Where?

[Kaz Kylheku]

On 2011-11-11, だいとうりょう むがべ <mugabeda...@gmail.com> wrote:
> I'm Japanese newbie. Excuse me in halting English.
> Please let me know.
> from CLHS. &OPTIONAL parameters.
> http://www.lispworks.com/documentation/HyperSpec/Body/03_daf.htm
>
> ((lambda (&optional (a 2 b) (c 3 d) &rest x) (list a b c d x)))
>=> (2 NIL 3 NIL NIL)
> a is 2.
> b is 3.
> other nil.OK

Hi,

I think you are mistaken here. A is 2, and C is 3; others NIL.

> ((lambda (&optional (a 2 b) (c 3 d) &rest x) (list a b c d x)) 6)
>=> (6 T 3 NIL NIL)
>
> T is the what?.

T stands for true. It is the value of B.

&optional (a 2 b) means this:

A : name of the paramter
2 : default value if argument is missing
B : name of a boolean parameter which indicates whether
the argument is present

B being T means "the caller passed an argument for A".

Think: what if the caller passes the value 2 for optional parameter A?
That is the same as the default value: ambiguous!

How can you tell if this 2 is the default 2, or the caller's 2?

The parameter B tells you. If it is T, then the argument is from
the caller. If it is NIL, then A's value was defaulted.

Cheers ...

[Pascal J. Bourguignon]

(mapcar (lambda (args)
(format t "~%Call with ~S~%" args)
(apply (lambda (&optional (a 2 ap) (c 3 cp) &rest x)
(format t "The caller ~:[didn't~;did ~] provide an argument for A~%" ap)
(format t "The value bound to the parameter A is ~S~%" a)
(format t "The caller ~:[didn't~;did ~] provide an argument for C~%" cp)
(format t "The value bound to the parameter C is ~S~%" c)
(format t "The caller ~:[didn't provide any other argument~;did provide other arguments: ~:*~S~]~%" x)
(list a ap c cp x))
args))
'( ()
(2 3)
(10)
(10 11)
(10 11 12)
(10 11 12 13)))



Call with NIL
The caller didn't provide an argument for A
The value bound to the parameter A is 2
The caller didn't provide an argument for C
The value bound to the parameter C is 3
The caller didn't provide any other argument

Call with (2 3)
The caller did provide an argument for A
The value bound to the parameter A is 2
The caller did provide an argument for C
The value bound to the parameter C is 3
The caller didn't provide any other argument

Call with (10)
The caller did provide an argument for A
The value bound to the parameter A is 10
The caller didn't provide an argument for C
The value bound to the parameter C is 3
The caller didn't provide any other argument

Call with (10 11)
The caller did provide an argument for A
The value bound to the parameter A is 10
The caller did provide an argument for C
The value bound to the parameter C is 11
The caller didn't provide any other argument

Call with (10 11 12)
The caller did provide an argument for A
The value bound to the parameter A is 10
The caller did provide an argument for C
The value bound to the parameter C is 11
The caller did provide other arguments: (12)

Call with (10 11 12 13)
The caller did provide an argument for A
The value bound to the parameter A is 10
The caller did provide an argument for C
The value bound to the parameter C is 11
The caller did provide other arguments: (12 13)

--> ((2 NIL 3 NIL NIL)
(2 T 3 T NIL)
(10 T 3 NIL NIL)
(10 T 11 T NIL)
(10 T 11 T (12))
(10 T 11 T (12 13)))


--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.