Identity function

[hsmyers]

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(define (return x) x)

Gives me a self-documenting way of avoiding both catch, throw and eval as well. Please criticize?

[Jeff]

How would that be used for that?

[hsmyers]

Like this

Code: Select all

(define (parse-pawn-move s n)
  (filter on-board?
    (if (black-to-move? n)
      (let (lst (list (next-diagl s)(next-diagr s)(next-row s)))
        (if (= "6" (row s))
          (append lst (list (next-row (next-row s))))
          (return lst)))
      (let (lst (list (prev-diagl s)(prev-diagr s)(prev-row s)))
        (if (= "4" (row s))
          (append lst (list (prev-row (prev-row s))))
          (return lst))))))

[Jeff]

This does the same thing:

Code: Select all

(define (parse-pawn-move s n)
  (filter on-board?
    (if (black-to-move? n)
      (let (lst (list (next-diagl s)(next-diagr s)(next-row s)))
        (if (= "6" (row s))
          (append lst (list (next-row (next-row s))))
          lst))
      (let (lst (list (prev-diagl s)(prev-diagr s)(prev-row s)))
        (if (= "4" (row s))
          (append lst (list (prev-row (prev-row s))))
          lst)))))

[hsmyers]

Why yes it does. Too much of non-functional approach. It at least worked; but yours requires less typing and one less function call.

[Jeff]

Return doesn't make sense in newlisp. The value of a call is the value of the last expression evaluated.

[hsmyers]

Hmmm, pretty sweeping statement there. That would imply that the idomatic use of catch and throw is never needed as well. I also remember seeing return in Common Lisp (granted as a macro), so while your statement is true for newLISP at least someone else found the need. I vaguely remember that MuLisp had one but I wouldn't swear to it. It was my first lisp back in the 80's (for an 8-bit machine) and I wrote several math packages for it--- seems like a 'return' would have been handy, but memory fades.

The value of a call is the value of the last expression evaluated.

Speaking of memory, thank you for reminding me of this little detail. And least I be accused of being original, here is a quote from Guy L. Steele Jr.

This function is occasionally useful as an argument to other functions that require functions as arguments (Got that?)

Kinda weak I'll admit--- but hey, what can I say!

[Jeff]

That would imply that the idomatic use of catch and throw is never needed as well.

That's not the same thing. If I want to break a loop:

Code: Select all

(set 'foo (catch (dotimes (i 10) (if (= i 5) (throw i)))))

That will give me 5. Catch also let's you work around error-prone routines (although many built-ins return nil and an error function in these cases).