There are two general mechanisms provided by this library that are used to build web scrapers: Selectors and Scrapers.
Selectors describe a location within an HTML DOM tree. The simplest selector,
that can be written is a simple string value. For example, the selector
"div" matches every single div node in a DOM. Selectors can be combined
using tag combinators. The
// operator to define nested relationships within a
DOM tree. For example, the selector
"div" // "a" matches all anchor tags
nested arbitrarily deep within a div tag.
In addition to describing the nested relationships between tags, selectors can
also include predicates on the attributes of a tag. The
@: operator creates a
selector that matches a tag based on the name and various conditions on the
tag’s attributes. An attribute predicate is just a function that takes an
attribute and returns a boolean indicating if the attribute matches a criteria.
There are several attribute operators that can be used to generate common
@= operator creates a predicate that matches the name and
value of an attribute exactly. For example, the selector
"div" @: ["id" @= "article"] matches div tags where the id attribute is equal to
Scrapers are values that are parameterized over a selector and produce a value
from an HTML DOM tree. The
Scraper type takes two type parameters. The first
is the string like type that is used to store the text values within a DOM tree.
Any string like type supported by
Text.StringLike is valid. The second type
is the type of value that the scraper produces.
There are several scraper primitives that take selectors and extract content from the DOM. Each primitive defined by this library comes in two variants: singular and plural. The singular variants extract the first instance matching the given selector, while the plural variants match every instance.
Complete examples can be found in the examples folder in the scalpel git repository.
The following is an example that demonstrates most of the features provided by
this library. Supposed you have the following hypothetical HTML located at
"http://example.com/article.html" and you would like to extract a list of all
of the comments.
<html> <body> <div class='comments'> <div class='comment container'> <span class='comment author'>Sally</span> <div class='comment text'>Woo hoo!</div> </div> <div class='comment container'> <span class='comment author'>Bill</span> <img class='comment image' src='http://example.com/cat.gif' /> </div> <div class='comment container'> <span class='comment author'>Susan</span> <div class='comment text'>WTF!?!</div> </div> </div> </body> </html>
The following snippet defines a function,
allComments, that will download
the web page, and extract all of the comments into a list:
type Author = String data Comment = TextComment Author String | ImageComment Author URL deriving (Show, Eq) allComments :: IO (Maybe [Comment]) allComments = scrapeURL "http://example.com/article.html" comments where comments :: Scraper String [Comment] comments = chroots ("div" @: [hasClass "container"]) comment comment :: Scraper String Comment comment = textComment <|> imageComment textComment :: Scraper String Comment textComment = do author <- text $ "span" @: [hasClass "author"] commentText <- text $ "div" @: [hasClass "text"] return $ TextComment author commentText imageComment :: Scraper String Comment imageComment = do author <- text $ "span" @: [hasClass "author"] imageURL <- attr "src" $ "img" @: [hasClass "image"] return $ ImageComment author imageURL
- Cleanup stale instance references in documentation of TagName and AttributeName.
- Made Scraper an instance of MonadPlus.
- Fixed examples in documentation and added an examples folder for ready to compile examples. Added travis tests to ensures that examples remain compilable.
- Removed the StringLike parameter from the Selector, Selectable, AttributePredicate, AttributeName, and TagName types. Instead they are now agnostic to the underlying string type, and are only constructable with Strings and the Any type.
- Tighten dependencies and drop download-curl all together.
- Add the html and html scraper primitives for extracting raw HTML.
- Make scrapeURL follow redirects by default.
- Expose a new function scrapeURLWithOpts that takes a list of curl options.
- Fix bug (#2) where image tags that do not have a trailing “/” are not selectable.
- Tighten dependencies on download-curl.
- First version!