🚀 go-pugleaf

    Thanks Saumya for pointing out some inefficiency in the code I had
    posted. And also for pointing out that checking for divisors upto
    "ceil(sqrt(N) + 1)" is sufficient to test for primeness.

    This property gives a better foundation to the latter code that
    I'd posted, where I was checking for divisor between I .. N//I+1,
    (should have read I .. ceil(N//I+1) I think the recursion
    terminates at ceil(sqrt(N)+1).

    And regarding my remark on O'K: It was a pun and not a sigh of
    relief. I have explicitly communicated my appreciation for his
    taking the time to comment on various issues including coding
    style in a very early posting, and also to him in person.

    And I also appreciate Saumya's comments. In particular the
    insights regarding choice point creation and the possibility of
    writing code that may enable compilers utilize architecture
    specific features.

    Now an offshoot thought.

    It seems to me that to understand optimization issues in Prolog
    knowing the architecure of WAM helps a lot. Besides choice point
    creation things like indexing on the first argument, bypassing the
    need to shuffle arguments by maintaining the same order for
    arguments in calls to other subprocedures, etc become clear only
    after one knows how Prolog is compiled to WAM. (Assuming WAM as
    the defacto standard abstract machine).

    To me these issues became clear after reading Gabriel et. al's 
    tutorial on WAM, and Warren's tutorial in Seattle. With the
    working knowledge of WAM that I have gathered, I can appreciate
    Saumya's comments.

    Now some queries: 

    Do people with little or no knowledge of WAM find it a bottleneck
    in gaining efficiency that is beyond gains achievable due to
    algorithmic changes or perturbations?

    Could one say, that the invisibility of control in Prolog is good
    for writing "declarative programs" but not good enough for writing
    efficient ones?

    In contrast, does the visibility of control in other programming
    languages like Pascal, C, etc help one write efficient programs?

    How long would it be when the issues like optimizing choice-points
    can be taken care of by the compiler itself? and at what
    cost/restriction to the programmer, if any?


Arun Lakhotia