Difference between revisions of "UrlDisp"
m (grammar fix) |
Ashalkhakov (talk | contribs) (more examples) |
||
Line 10: | Line 10: | ||
UrlDisp provides (Fast)CGI programs a minimalistic domain-specific parser for URLs. |
UrlDisp provides (Fast)CGI programs a minimalistic domain-specific parser for URLs. |
||
− | Hierarchical part of the URL is tokenized and matched against rules defined using UrlDisp combinators. Every rule consists of, basically, a predicate and a CGI action. Once a predicate is satisfied, an action is performed; otherwise, alternatives are tried in order. The matching algorithm is backtracking. |
+ | Hierarchical part of the URL (e.g., /foo/bar/ or /bar/baz/quix) is tokenized (turned into a list of "URL fragments", e.g. ["foo","bar"]) and matched against rules defined using UrlDisp combinators. Every rule consists of, basically, a predicate and a CGI action. Once a predicate is satisfied, an action is performed; otherwise, alternatives are tried in the order given (<|> associates to the left). The matching algorithm is backtracking. |
== Usage examples == |
== Usage examples == |
||
Line 20: | Line 20: | ||
<hask>output "hello, world!"</hask> |
<hask>output "hello, world!"</hask> |
||
+ | This one replies to all requests with "hello, world!". |
||
− | Adding a predicate: |
||
+ | |||
+ | One can add a predicate to make things more interesting: |
||
<hask> |
<hask> |
||
− | -- if URL matches /hello, then output "hello, world!" |
||
h |/ "hello" *> output "hello, world!"</hask> |
h |/ "hello" *> output "hello, world!"</hask> |
||
+ | This one will greet people only if the URL starts with "/hello". It will give a 404 error page otherwise. |
||
− | More examples: |
||
+ | |||
+ | Such "if-then" clauses can be combined using "or" -- (<|>), which associates to the left, so |
||
+ | |||
+ | <hask>a <|> b <|> c</hask> |
||
+ | |||
+ | is equivalent to |
||
+ | |||
+ | <hask>((a <|> b) <|> c)</hask> |
||
+ | |||
+ | Anyway, code using UrlDisp shouldn't depend on this property. |
||
+ | |||
+ | To introduce an "and" in your rule, apply (|/) successively, as in |
||
+ | |||
+ | <hask>h |/ "foo" |/ "bar"</hask> |
||
+ | |||
+ | Generally, other combinators will correspond to "and" and bind stronger than "or". For example: |
||
<hask> |
<hask> |
||
⚫ | |||
− | -- if URL contains /hello, output "woot, it works!", otherwise check for |
||
+ | <|> endPath |? ("cmd, "bar") *> output "goodbye" |
||
− | -- /foo |
||
⚫ | |||
</hask> |
</hask> |
||
+ | |||
+ | Will behave as follows: |
||
+ | * all GET requests to /foo/bar (and anything that follows) and parameter "cmd" set to "foo" will output "hello" |
||
+ | * requests with empty path and parameter cmd set to "bar" will output "goodbye" |
||
+ | * other requests will trigger a 404 page |
||
As you can see, the |/ combinator matches current token against its right operand. h is a special predicate that matches anything, it is used to begin a string of combinators. |
As you can see, the |/ combinator matches current token against its right operand. h is a special predicate that matches anything, it is used to begin a string of combinators. |
||
Line 45: | Line 66: | ||
=== Extending UrlDisp === |
=== Extending UrlDisp === |
||
− | The examples given above are not very interesting since one wants to interact with outside world. Let's |
+ | The examples given above are not very interesting since one wants to interact with the outside world. Let's take a look at how to extend UrlDisp to handle database access. |
Wrapping UrlDisp around a ReaderT will do the trick. |
Wrapping UrlDisp around a ReaderT will do the trick. |
||
Line 71: | Line 92: | ||
m :: UrlDisp (ReaderT Connection (CGIT IO)) CGIResult |
m :: UrlDisp (ReaderT Connection (CGIT IO)) CGIResult |
||
m = do |
m = do |
||
− | v <- lift ask >>= \c -> liftIO (quickQuery' c |
+ | v <- lift ask >>= \c -> liftIO (quickQuery' c queryText []) |
output $ show v |
output $ show v |
||
+ | |||
+ | -- you will have to provide this one |
||
+ | queryText = "select * from ..." |
||
</hask> |
</hask> |
||
Revision as of 07:30, 26 May 2009
What is UrlDisp
Problem statement
URLs are everywhere on the web. Most of them, however, are hard to remember, because they are meaningless for humans. This is wrong: URLs are a part of user interface, and therefore should be kept simple, meaningful and memorizeable.
Solution
UrlDisp provides (Fast)CGI programs a minimalistic domain-specific parser for URLs.
Hierarchical part of the URL (e.g., /foo/bar/ or /bar/baz/quix) is tokenized (turned into a list of "URL fragments", e.g. ["foo","bar"]) and matched against rules defined using UrlDisp combinators. Every rule consists of, basically, a predicate and a CGI action. Once a predicate is satisfied, an action is performed; otherwise, alternatives are tried in the order given (<|> associates to the left). The matching algorithm is backtracking.
Usage examples
Basics
A regular CGI action looks like this:
output "hello, world!"
This one replies to all requests with "hello, world!".
One can add a predicate to make things more interesting:
h |/ "hello" *> output "hello, world!"
This one will greet people only if the URL starts with "/hello". It will give a 404 error page otherwise.
Such "if-then" clauses can be combined using "or" -- (<|>), which associates to the left, so
a <|> b <|> c
is equivalent to
((a <|> b) <|> c)
Anyway, code using UrlDisp shouldn't depend on this property.
To introduce an "and" in your rule, apply (|/) successively, as in
h |/ "foo" |/ "bar"
Generally, other combinators will correspond to "and" and bind stronger than "or". For example:
h |// "GET" |/ "foo" |/ "bar" |? ("cmd", "foo") *> output "hello" <|> endPath |? ("cmd, "bar") *> output "goodbye"
Will behave as follows:
- all GET requests to /foo/bar (and anything that follows) and parameter "cmd" set to "foo" will output "hello"
- requests with empty path and parameter cmd set to "bar" will output "goodbye"
- other requests will trigger a 404 page
As you can see, the |/ combinator matches current token against its right operand. h is a special predicate that matches anything, it is used to begin a string of combinators.
One can also match against
- URL parameters,
- HTTP methods,
- and also convert token into a variable which is an instance of Read
There's also an API which is believed to be more human-readable.
Extending UrlDisp
The examples given above are not very interesting since one wants to interact with the outside world. Let's take a look at how to extend UrlDisp to handle database access.
Wrapping UrlDisp around a ReaderT will do the trick.
{-# LANGUAGE FlexibleInstances, ScopedTypeVariables #-} import Network.UrlDisp import Database.HDBC import Database.HDBC.ODBC import Control.Exception (bracket) import Network.CGI import Network.CGI.Monad instance MonadCGI (ReaderT Connection (CGIT IO)) where cgiAddHeader n v = lift $ cgiAddHeader n v cgiGet = lift . cgiGet -- once a request to "/db/" is sent, -- execute an SQL query and show its results main :: IO () main = bracket (connectODBC connStr) disconnect (\c -> runCGI $ (flip runReaderT) c $ evalUrlDisp $ ((h |/ "db" *> m) <|> output "not found")) m :: UrlDisp (ReaderT Connection (CGIT IO)) CGIResult m = do v <- lift ask >>= \c -> liftIO (quickQuery' c queryText []) output $ show v -- you will have to provide this one queryText = "select * from ..."