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Forum => So, what can you actually DO with newLISP? => Topic started by: ssqq on November 03, 2014, 04:40:51 AM

Title: How to call function each other with declaration
Post by: ssqq on November 03, 2014, 04:40:51 AM
If I design a function, need call other function which also call this function. How to Decalare it:



            (define (sub-1 arg)

                (if (> arg 1) (sub-2 arg)))



            (define (sub-2 arg)

                (if (< arg 1) (sub-1 arg)))
Title: Re: How to call function each other with declaration
Post by: ryuo on November 03, 2014, 09:41:02 AM
Something like this?


Code Select

(define (foo x)
(if (> x 0)
(bar x)
(- x 1)
)
)
(define (bar x)
(if (< x 0)
(foo x)
(+ x 1)
)
)
Title: Re: How to call function each other with declaration
Post by: rickyboy on November 03, 2014, 10:55:51 AM
Quote from: "ssqq"If I design a function, need call other function which also call this function. How to Decalare it: . . .

What you wrote (or what ryuo wrote) is the basic idea.  It is OK because the calls of sub-1 and sub-2 in the bodies of the functions (lambdas) only get resolved during call time.  For instance, the call (sub-2 arg) in the body of the definition of sub-1 is not illegal (ostensibly because sub-2 hasn't been defined yet), because the symbol sub-2 in the expression (sub-2 arg) gets resolved *only* when the expression (sub-2 arg) is called, and when that happens, the function sub-2 will already have been defined.  I hope that's clear.



The only thing you might have to be concerned about is blowing the stack, because mutual recursion is not stack optimized in newLISP.  But, neither is any recursion.  For instance, take the classic example of mutual recursion in SICP.


Code Select
(define (ev? n)
  (if (= n 0)
      true
      (od? (- n 1))))

(define (od? n)
  (if (= n 0)
      false
      (ev? (- n 1))))

(SICP actually uses the names even? and odd?, but those are already protected names in newLISP.)



Your can see that works quite well.


Code Select
> (od? 55)
true
> (ev? 55)
nil
> (od? 42)
nil
> (ev? 42)
true

But the recursive calls do grow the call stack, and there will always be a limit to that.


Code Select
> (ev? 2349)

ERR: call or result stack overflow in function if : =
called from user defined function od?
called from user defined function ev?
called from user defined function od?
called from user defined function ev?
called from user defined function od?
called from user defined function ev?
called from user defined function od?
. . .
Title: Re: How to call function each other with declaration
Post by: ssqq on November 03, 2014, 10:38:43 PM
Thanks all, I have not test this case. Because I found newlisp must decalare the function in advance before use it. So I think this case may throw a error.
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