Introduction to Haskell Diagrams

I need a solid declarative diagramming library or tool that I can invest time in. I always wanted to learn Haskell’s diagrams library. In this post, I will give it a try.

Motivation

I like drawing. Most people I know are visual people. I decided to find a decent diagramming tool which I can integrate in both my workflow and programs.

I can go two ways:

1. Use a low-level language like , which are good for the purpose they are designed for, but not for generic purpose diagramming.

I think that I need something in between. Recently, I was checking Haskell library for a while. I know, it is quite low-level to use for occassional reasons, but high-level and generic enough to use in a program. It is time to give it a try.

Getting Our Hands Dirty

This blogpost is written in Literate Haskell. All the images produced in this post are generated by this post itself evertime I (or GitHub Action) build this blog. You can check the source code of my blog.

Let’s start…

I am simply going to add diagrams to my Haskell dependencies along with the markdown-unlit program.

{

  ##...



  ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: [

    ## ...

    hpkgs.diagrams

    hpkgs.markdown-unlit

    ## ...

  ]);



  thisShell = pkgs.mkShell {

    buildInputs = [

      ## ...



      ghc



      ## ...

    ];



    NIX_GHC = "${ghc}/bin/ghc";

    NIX_GHCPKG = "${ghc}/bin/ghc-pkg";

    NIX_GHC_DOCDIR = "${ghc}/share/doc/ghc/html";

    NIX_GHC_LIBDIR = "${ghc}/lib/ghc-9.6.5/lib";

  };



  # ...

}

diagrams tutorial says that we may need to enable some language extensions. But we will not need them in this port. We just need some imports:

import Diagrams.Backend.SVG

import Diagrams.Prelude

import System.Environment (getArgs)

We will generate 11 SVG diagrams in this tutorial. Let’s implement our entry point:

main :: IO ()

main = do

  dir <- head <$> getArgs

  render dir "diagram1.svg" diagram1

  render dir "diagram2.svg" diagram2

  render dir "diagram3.svg" diagram3

  render dir "diagram4.svg" diagram4

  render dir "diagram5.svg" diagram5

  render dir "diagram6.svg" diagram6

  render dir "diagram7.svg" diagram7

  render dir "diagram8.svg" diagram8

  render dir "diagram9.svg" diagram9

  render dir "diagram10.svg" diagram10

  render dir "diagram11.svg" diagram11

  where

    render dpath fname = renderSVG (dpath <> "/" <> fname) (mkSizeSpec2D (Just 400) Nothing) . frame 0.2

This is how we will invoke it:

runhaskell \

  -pgmLmarkdown-unlit \

  content/posts/2024-08-09_haskell-diagrams-intro.lhs \

  static/assets/media/posts/haskell-diagrams-intro

Let’s start with a triangle:

diagram1 :: Diagram B

diagram1 =

  triangle 1

How about putting a list of shapes side by side separated by a gap?

diagram3 :: Diagram B

diagram3 =

  hsep 0.2 $

    [ triangle 1

    , triangle 1

    ]

It works for the shapes composed together as well:

diagram5 :: Diagram B

diagram5 =

  showOrigin . hsep 0.2 $

    [ triangle 1

    , triangle 1

    ]

How about adding a vertical line between the triangles?

diagram7 :: Diagram B

diagram7 =

  centerX . hsep 0.2 $

    [ triangle 1

    , vrule 1

    , triangle 1

    ]

Of course! The origin along the y-axis is anchored to the first shape’s origin that is a triangle. And the center of this equilateral triangle is not the center of its height. See your nearest analytical geometry book for details!

Let’s align the shapes at to the top of the shapes:

diagram9 :: Diagram B

diagram9 =

  centerX . (composeAligned alignT (hsep 0.2)) $

    [ triangle 1

    , vrule 1

    , triangle 1

    ]

OK, now the final diagram attempt which will render some text inside the triangles:

diagram11 :: Diagram B

diagram11 =

  hrule 1.4 === hcom

  where

    work = triangle 1 === scale 0.15 (alignedText 0.5 0 "work")

    life = triangle 1 === scale 0.15 (alignedText 0.5 0 "life")

    hcom = centerX . (composeAligned alignT (hsep 0.2)) $ [work, vrule 1, life]

Wrap-Up

In this tutorial, we glimpsed through the basics of the diagrams library.

Now that we have established work-life balance, I can get ready for the next chapter of my life with diagrams.