I apologize for the bad title, but I don't quite know how to summarize this.
I'm a Haskell beginner, making a website in Haskell (Hakyll) using templates in Blaze-HTML, which uses do notation to make HTML. Here's part of my template:
defaultTemplateRaw :: Html
defaultTemplateRaw = html $ do
H.head $ do
meta ! httpEquiv "Content-Type" ! content "text/html; charset=UTF-8"
H.title "My Hakyll Blog - $title$"
link ! rel "stylesheet" ! type_ "text/css" ! href "/css/main.css"
link ! rel "stylesheet" ! type_ "text/css" ! href "https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
link ! rel "stylesheet" ! type_ "text/css" ! href "https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css"
Now let's say I wanted to be DRY here, so I wouldn't have all the link
s, just a list of URLs in some kind of list comprehension. How would I do that?
You can use mapM_
here:
thelinks :: [AttributeValue]
thelinks = ["list", "of", "hrefs"]
defaultTemplateRaw :: Html
defaultTemplateRaw = html $ do
H.head $ do
meta ! httpEquiv "Content-Type" ! content "text/html; charset=UTF-8"
H.title "My Hakyll Blog - $title$"
mapM_ ((link ! rel "stylesheet" ! type_ "text/css" !) . href) thelinks
So here we use mapM_ :: Monad m => (a -> m b) -> t a -> m ()
to process the monadic function ((link ! rel "stylesheet" ! type_ "text/css" !) . href)
for every element of the list thelinks
.
We construct the function by using operator section, so:
(link ! rel "stylesheet" ! type_ "text/css" !)
is equivalent with:
\x -> link ! rel "stylesheet" ! type_ "text/css" ! x
But we can not directly pass an AttributeValue
as x
to that function: we need to use the href
attribute, we do so by using . href
, thus thus means that:
(link ! rel "stylesheet" ! type_ "text/css" !) . href
-----------------------------------------------------------------------
= (\x -> link ! rel "stylesheet" ! type_ "text/css" ! x) . href
-----------------------------------------------------------------------
= \y -> (\x -> link ! rel "stylesheet" ! type_ "text/css" ! x) (href y)
-----------------------------------------------------------------------
= \y -> (link ! rel "stylesheet" ! type_ "text/css" ! (href y))
So it is a more syntactically convenient way to call the href
function on the item of the list, and that result in the structure of link ! ...
.
For the given sample list, this produces:
Main L R> putStrLn $ L.unpack $ R.renderHtml defaultTemplateRaw
<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>My Hakyll Blog - $title$</title><link rel="stylesheet" type="text/css" href="list"><link rel="stylesheet" type="text/css" href="of"><link rel="stylesheet" type="text/css" href="hrefs"></head></html>