Handling errors and multiple files
We have left an unimplemented function last chapter, and there are a few more things left for us to do to actually call our program a static blog generator. We still need to process multiple files in a directory and create an index landing page with links to other pages.
Links in HTML
Our HTML EDSL currently does not support links or other content modifiers such as bold and italics. We should add these so we can use them when creating an index.
Up until now we've passed
Structure creating functions such as
h_. Instead, we could create and pass them a new type,
can be regular text, links, images, and so on.
Exercise: implement what we've just discussed. Follow the compiler errors and refactor what needs refactoring.
module HsBlog.Html.Internal where import Numeric.Natural -- * Types newtype Html = Html String newtype Structure = Structure String newtype Content = Content String type Title = String -- * EDSL html_ :: Title -> Structure -> Html html_ title content = Html ( el "html" ( el "head" (el "title" (escape title)) <> el "body" (getStructureString content) ) ) -- * Structure p_ :: Content -> Structure p_ = Structure . el "p" . getContentString h_ :: Natural -> Content -> Structure h_ n = Structure . el ("h" <> show n) . getContentString ul_ :: [Structure] -> Structure ul_ = Structure . el "ul" . concat . map (el "li" . getStructureString) ol_ :: [Structure] -> Structure ol_ = Structure . el "ol" . concat . map (el "li" . getStructureString) code_ :: String -> Structure code_ = Structure . el "pre" . escape instance Semigroup Structure where (<>) c1 c2 = Structure (getStructureString c1 <> getStructureString c2) instance Monoid Structure where mempty = Structure "" -- * Content txt_ :: String -> Content txt_ = Content . escape link_ :: FilePath -> Content -> Content link_ path content = Content $ elAttr "a" ("href=\"" <> escape path <> "\"") (getContentString content) img_ :: FilePath -> Content img_ path = Content $ "<img src=\"" <> escape path <> "\">" b_ :: Content -> Content b_ content = Content $ el "b" (getContentString content) i_ :: Content -> Content i_ content = Content $ el "i" (getContentString content) instance Semigroup Content where (<>) c1 c2 = Content (getContentString c1 <> getContentString c2) instance Monoid Content where mempty = Content "" -- * Render render :: Html -> String render html = case html of Html str -> str -- * Utilities el :: String -> String -> String el tag content = "<" <> tag <> ">" <> content <> "</" <> tag <> ">" elAttr :: String -> String -> String -> String elAttr tag attrs content = "<" <> tag <> " " <> attrs <> ">" <> content <> "</" <> tag <> ">" getStructureString :: Structure -> String getStructureString structure = case structure of Structure str -> str getContentString :: Content -> String getContentString content = case content of Content str -> str escape :: String -> String escape = let escapeChar c = case c of '<' -> "<" '>' -> ">" '&' -> "&" '"' -> """ '\'' -> "'" _ -> [c] in concat . map escapeChar
module HsBlog.Html ( Html , Title , Structure , html_ , p_ , h_ , ul_ , ol_ , code_ , Content , txt_ , img_ , link_ , b_ , i_ , render ) where import HsBlog.Html.Internal
module HsBlog.Convert where import qualified HsBlog.Markup as Markup import qualified HsBlog.Html as Html convert :: Html.Title -> Markup.Document -> Html.Html convert title = Html.html_ title . foldMap convertStructure convertStructure :: Markup.Structure -> Html.Structure convertStructure structure = case structure of Markup.Heading n txt -> Html.h_ n $ Html.txt_ txt Markup.Paragraph p -> Html.p_ $ Html.txt_ p Markup.UnorderedList list -> Html.ul_ $ map (Html.p_ . Html.txt_) list Markup.OrderedList list -> Html.ol_ $ map (Html.p_ . Html.txt_) list Markup.CodeBlock list -> Html.code_ (unlines list)
You can view the git commit of the changes we've made and the code up until now.
Creating an index page
With our extended HTML EDSL, we can now create an index page with links to the other pages.
To create an index page, we need a list of files with their target destinations,
as well as their
Markup (so we can extract information to include in our index page,
such as the first heading and paragraph). Our output should be an
We need to implement the following function:
buildIndex :: [(FilePath, Markup.Document)] -> Html.Html
buildIndex :: [(FilePath, Markup.Document)] -> Html.Html buildIndex files = let previews = map ( \(file, doc) -> case doc of Markup.Heading 1 heading : article -> Html.h_ 3 (Html.link_ file (Html.txt_ heading)) <> foldMap convertStructure (take 3 article) <> Html.p_ (Html.link_ file (Html.txt_ "...")) _ -> Html.h_ 3 (Html.link_ file (Html.txt_ file)) ) files in Html.html_ "Blog" ( Html.h_ 1 (Html.link_ "index.html" (Html.txt_ "Blog")) <> Html.h_ 2 (Html.txt_ "Posts") <> mconcat previews )
Our general strategy for processing whole directories is going to be:
- Create the output directory
- Grab all file names in a directory
- Filter them according to their extension, we want to process
txtfile and copy other files without modification
- We want to parse each text file, build an index of the result, convert the files to HTML, and write everything to the target directory
While our parsing function can't really fail, trying to read or write a file to the file-system can fail in several ways. It would be nice if our static blog generator was robust enough that it wouldn't fail completely if one single file gave it some trouble. This is a good opportunity to learn about error handling in Haskell, both in uneffectful code and for I/O code.
In the next few chapters we'll survey the landscape of error handling in Haskell before figuring out the right approach for our use case.