speculative sketch of a web scraper in Prolog with XPath, regular expressions and constraints

scrape.prolog

% The numerous inline comments are explanatory.  Normal code wouldn't have them.
% I'm using double quotes to delineate raw text content.  Real code would use
% a quasiquotation mechanism.

amazon_condition_overview(ASIN, Condition, BestPrice, Inventory, Section) :-
    % HTTP GET the URL and parse it into a DOM structure
    get(url("amazon.com/gp/offer-listing/$ASIN/?ie=UTF8"), DOM),

    % build an atom representing the tab's node ID
    TabId = qq("olpTab$Condition"),

    % constrain From to only those values which this DCG can parse
    phrase(from_lowest_price(BestPrice), From),

    % describe where the relevant DOM nodes are and their relation to each other
    DOM ~~ xpath("//ol[@id='olpTabs']/li[@id=$TabId]/a/span[@class='numberreturned'][.=$From]"),

    % the h2 tag content matches this regular expression.
    % interpolating Condition escapes any internal meta characters (including spaces)
    H2 ~~ rx("^ \s* ${Condition} \b", ix),

    % extract the number of offers using a named capture
    OfferCount ~~ rx("of \s+ (?P<InventoryAtom>\d+) \s+ offer", ix),
    atom_number(InventoryAtom, Inventory),

    % constrain IsOffer to be an HTML attribute containing the class "olpOffer"
    has_class(IsOffer, olpOffer),

    % find the relevant section of the DOM
    DOM ~~ xpath("//div[//h2[.=$H2]/span[.=$OfferCount]][//div[@class=$IsOffer]]", Section).

from_lowest_price(Price) -->
    "from ",
    dollars_and_cents(Pennies).

dollars_and_cents(Pennies) -->
    "$",
    integer(Dollars),
    ".",
    integer(Cents),
    Pennies is 100*Dollars + Cents.