This blog post is a Literate Haskell program that produces its own OpenGraph image using the infamous Haskell diagrams library.
Motivation
So far, I have written a few blog posts about generating images with Haskell's diagrams library. In this blog post, I will try to combine these blog posts into a superpower: Generating OpenGraph images dynamically including the title and description of the blog post.
Program
Before we start, one important warning: You may find the code below not very well composed. The reason is that I am still getting to know the diagrams library and its capabilities. I am also limiting my time to write these blog posts. Eventually, I will get better at both and revisit these blog posts.
We will use the 4 Haskell packages for this Literate Haskell program like before: diagrams, diagrams-cairo, SVGFonts and markdown-unlit.
Let's import our modules of interest:
import Diagrams.Backend.Cairo
import Diagrams.Prelude
import qualified Graphics.SVGFonts as F
import qualified Graphics.SVGFonts.Wrap as F.Wrap
import Data.Maybe (fromMaybe)
import System.Environment (getArgs)
Let's implement our entry point what will read the command line argument for output directory, my Website logo and render the OpenGraph image:
main :: IO ()
main = do
dir <- head <$> getArgs
logo <- loadLogo
render dir "og.png" (og logo "thenegation.com" "@vst" title description)
where
render dpath fname =
renderCairo (dpath <> "/" <> fname) (mkSizeSpec2D (Just 1200) (Just 630))
This is how we will run our blog post:
runhaskell \
-pgmLmarkdown-unlit \
content/posts/2024-08-14_haskell-diagrams-dynamic-og.lhs \
static/assets/media/posts/haskell-diagrams-dynamic-og
Now, the first thing we need is the OpenGraph title and description. Normally, we would parse it from the YAML metadata (or front-matter) of the Markdown file. But, let's hard-code it to focus into the diagrams part in this blog post:
title :: String
title =
"More Haskell Diagrams: Dynamic OpenGraph Images"
description :: String
description =
"This blog post is a Literate Haskell program that produces its own OpenGraph image using the infamous Haskell diagrams library."
We will also need the Website logo:
loadLogo :: IO (Diagram B)
loadLogo = do
(Right img) <- loadImageEmb "./static/android-chrome-512x512.png"
pure $ scaleUToX 10 $ image img
We will compose the OpenGraph as 3 different segments:
- The title
- The description
- The Website logo, URL and author
Both title and description can be long text, and we may need to wrap them. Here is our function to do it:
textWrapped :: Double -> (Double, Double) -> String -> Diagram B
textWrapped height bounds txt =
final <> boundingRect (frame 0 final) # lwO 0 # value mempty
where
mLines = F.Wrap.wrapText def height [(F.Wrap.splitAtSpaces, bounds)] txt
putTxt = lw none . fc black . F.set_envelope . F.svgText def
final = vsep 0.2 (fmap putTxt (fromMaybe ["what happened?"] mLines))
Let's prepare the title:
ogiTitle :: String -> Diagram B
ogiTitle t =
textWrapped 1 (14, 16) t
Now, the description:
ogiDescription :: String -> Diagram B
ogiDescription d =
textWrapped 1 (20, 25) d
The footer is going to contain the website logo, URL and author:
ogiFooter :: Diagram B -> String -> String -> Diagram B
ogiFooter l u a =
hsep 0 $
[ center $ clipBy (circle 5) l
, center $ rect 2 1 # lw none
, center $ u # F.svgText def # F.fit_width 40 # F.set_envelope # lw none # fc black
, center $ rect 58 1 # lw none
, center $ a # F.svgText def # F.fit_width 10 # F.set_envelope # lw none # fc black
]
Let's put them together. Note how we rescale both title and description to 120 units first:
og :: Diagram B -> String -> String -> String -> String -> Diagram B
og l u a t d =
(scaleUToX 110 $ center final) <> (rect 120 63 # bg (sRGB24read "#e4e4e7") # lw none)
where
content =
center . vsep 1 $
[ centerXY $ scaleUToX 120 $ ogiTitle t
, centerXY $ scaleUToX 120 $ ogiDescription d
]
footer = centerXY $ ogiFooter l u a
final = (content `atop` (rect 120 53 # lw none)) === footer
Wrap-Up
For a long time, I wanted to play with the diagrams library. I am glad that I finally did it with this and the previous blog posts. I understood the library good enough to create an OpenGraph image dynamically.
I think that there are some things quite difficult or tedious to achieve with the diagrams library. But, I also think that most of this difficulty is because of my lack of experience with the library. I am looking forward to getting better at it.
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