[Haskell-cafe] Re: Graphical representation of Haskell code

Maciej Piechotka uzytkownik2 at gmail.com
Wed Mar 24 22:37:28 EDT 2010

On Wed, 2010-03-24 at 21:33 -0400, Ronald Guida wrote:
> Those are some very interesting visual languages, Miguel!
> I remember drawing some diagrams when I was teaching myself Haskell,
> but I never actually tried to create a formal visual language.  Since
> my background is in hardware engineering, I would naturally gravitate
> toward schematic diagrams.  I am also familiar with the graphical
> programming language of LabView.
> After reading Miguel's exposition, I thought about how I might draw a
> picture of map1.
> map1 :: (a -> b) -> [a] -> [b]
> map1 f [] = []
> map1 f (x:xs) = (f x) : map1 f xs
> map1.png
> (Image created with Inkscape)
> Here is what I'm thinking:
> * Green boxes represent inputs and outputs.
> * Inputs are always on the left, outputs are always on the right.
> * Inputs appear in top-to-bottom order.
> * Data always flows left to right.
> * Arrows represent data flow.
> * A named white rectangle represents application of the named
> function.
> * A gray rectangle represents a function that arrives through data
> flow.
> * A filled-in arrowhead means the data "in" the arrow is to be
> "unpacked" and used as a function.
> * A named light-blue rectangle (such as on the left, with a colon in
> it) represents a pattern match operation.
> In thinking about this, I can sense that there are MANY issues with
> using a visual language to represent Haskell.  Some issues I can think
> of:
> * How to represent pattern matching?
> * How to represent partial application?
> * How to represent data types or class constraints in the diagram?
> * How to represent a list comprehension or a do statement? (These
> might require special visual syntax)
> * Will the data flow always take the form of a directed acyclic graph,
> so that data never has to flow right-to-left?  (Perhaps there's a way
> to "tie the knot" and get a cycle in the data flow graph.)

myfix f = let x = f x in x

f -------\
      |       |

Unless we rewrite it into:

myfix' f = f (myfix' f)

f ----+-------------------\
      |                   |
      |                   v
      \->X myfix' X----->X-X---->

> * Whether to create special symbols for certain commonly used
> functions? (In digital circuit schematics, AND, OR, and NOT gates have
> special symbols, but most compound circuits are represented with
> labeled rectangles.)
> Also, if I want to automatically generate an image from a Haskell
> function, then my image generator needs to automatically place and
> route all those boxes.
> I'll have to give more thought to the other versions of map, and maybe
> make some more diagrams.
> -- Ron

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