In the previous part, we ended with the following types:
+ + +> data OurSite +> = HomePage +> | MyGallery Gallery +> | YourGallery Gallery +> deriving (Read, Show) + +> data Gallery +> = Thumbnails +> | ShowImage Int Size +> deriving (Read, Show) + +> data Size +> = Full +> | Screen +> deriving (Read, Show) + +The problems we faced were:
+showLink was only called using
+ data-types that were actually handled by the sitegallery module in a larger sitegallery don't
+ collide.To solve the first problem, we need to restrict the types that
+ showLink can be applied to. To solve the second two
+ problems, we need some way to record the current context so that we
+ can use it when we generate the Link.
The perfect tool for this job is the Reader monad (also known as + the Enviroment monad):
+ +> type Link = String +> type LinkM link a = Reader (link -> Link) a + +We store a function in the enviroment which can be used to turn a
+ data type into a Link. The showLink
+ function is now parameterized over this monad as follows:
+
Now showLink can only be called on types supported by the
+ current environment. As a practical example, our gallery
+ function will now looks like this:
+
+ So, now if we tried to call showLink True inside gallery
+ we would get an error like:
+
/home/stepcut/n-heptane/projects/haskell/urlt/tutorial/SimpleSite2.lhs:81:26: + Couldn't match expected type `Gallery' against inferred type `Bool' + In the first argument of `showLink', namely `True' + In a 'do' expression: img1 <- showLink True + In the expression: + do img1 <- showLink True + return + $ pageTemplate + ((toHtml $ "Showing " ++ username ++ "'s gallery thumbnails.") + +++ br +++ (anchor (toHtml "image 1") ! [href img1])) +Failed, modules loaded: none.+ +
The LinkM monad also provides us with a convenient way to embed
+ the gallery library into a larger site:
+
Here is how we would use nestLink to support OurSite
Note that in the alternative for HomePage we create
+ the links to the galleries two different ways. But in both instances
+ the type of the link is OurSite.
More interesting is the alternatives which match on
+ MyGallery and YourGallery. To nest the
+ gallery we just use nestLink MyGallery and
+ nestLink YourGallery. The gallery library
+ itself needs no additional adjustments.
Additionally, let's say we forgot the nestLink + in the last alternative and instead write:
+ +ourSite (YourGallery g) = + gallery "someone else" g+ +
At compile time we will get an error like:
+ +/home/stepcut/n-heptane/projects/haskell/urlt/tutorial/SimpleSite2.lhs:134:6: + Couldn't match expected type `OurSite' + against inferred type `Gallery' + Expected type: Reader (OurSite -> Link) Html + Inferred type: LinkM Gallery Html + In the expression: gallery "someone else" g + In the definition of `ourSite': + ourSite (YourGallery g) = gallery "someone else" g +Failed, modules loaded: none.+ +
With the addition of the LinkM monad, we have neatly addressed the + goals set forth. We have leveraged the type-checker to ensure that + all the internal links are associated with some code that handles + them. Additionally, we can import third-party modules into a larger + site, and the generate links are automatically adjusted to match the + site structure. We can use the same 3rd-party module in more than one + place and the links will not collide. +
+ +The implementation presented in this part is a bit limiting, + because we can not use the IO monad or other similar Monads our site + function. In the next part we will make some trivial modifications to + use the Reader Monad Transformer instead of the plain Reader Monad.
+ +The Links generated by this code are very scary
+ looking. In the next part, we would show how to make prettier looking
+ links.
The type-safety of the Links is in part due to the + fact that the argument to the handler function is the same as the + type of link the monad is parameterized over. For example:
+ +ourSite :: OurSite -> LinkM OurSite Html+ +
One way to enforce this required is to code it into the type which + represents our site:
+ +> +> data Site link a +> = Site { handleLink :: link -> LinkM link a +> , defaultPage :: link +> , formatLink :: link -> Link +> , parseLink :: Link -> Maybe link +> } + +> ourSiteSpec :: Site OurSite Html +> ourSiteSpec = +> Site { handleLink = ourSite +> , defaultPage = HomePage +> , formatLink = escapeURIString isUnescapedInURI . show +> , parseLink = readLink +> } + + +Alternatively, we might choose to encode it in the LinkM type directly:
+ +> type LinkM' link a = link -> Reader (link -> Link) a + +However, I find that confusing. Consider:
+ +> testFunc :: String -> LinkM' link link +> testFunc str lnk = return lnk + +The type appears to indicate that testFunc takes one argument instead of two.
+ +Also, we can not write the type signature for nestLink using the
+ LinkM' type synonym. For these reasons I prefer the
+ first option.
In part III, we will make some small, final adjustments to + make the library more usuable for real world projects
+ +