i-am-tom/purescript-spirographs

CodeMesh 2018 - An introduction to PureScript canvas rendering and the Behaviors library.

---

title: Spirographs author: "@am\_i\_tom" patat: incrementalLists: true

images: backend: auto

wrap: true

margins: left: 10 right: 10

pandocExtensions: - emoji - patat_extensions ...

🎨 PureScript Spirographs 💻

Who am I? ✨

• 🎩 Tom Harding

• 👔 Habito (always hiring!)

• 💬 twitter.com/am_i_tom

• 🌱 github.com/i-am-tom

• 📚 tomharding.me

Spirographs 🌀

How does it work? 💡

• Start with a (fixed) circle (as a perimeter).

• Pick a point on a smaller, rolling circle.

• Roll the second circle around the edge of the first.

• Trace the path of the chosen point.

• Repeat until Mum's off the phone.

Risky Live Moment 1: Spirograph GIF 🙈

PureScript

In ASCII art? 👾

              .              'kKd'              
             'okkkkOOOOOOx:. .,xXXd'            
         ..   .:llllllllllc'   .,xXXd'          
       .o0k'                     .xWNo.         
     .oKNk;. .;dxxxxxxxxxxo'   .c0NO:.          
   .oKNk;.   'cooooooooooc'  .c0NO:.            
  .oWWx.                     'dk:.              
   .c0NO:.   'lddddddddddo,.   .                
     .c0NO:. .,lddddddddddl'                    
       .cOx'                                    
         ..                                     

In code? ⛵

module Main where

import Prelude
import Effect (Effect)
import Effect.Console (log)

main :: Effect Unit
main = do
  log "Hello sailor!"

Maths 💯

Let's write a type! ✏️

newtype Coordinate 
  = Coordinate
      { x :: Number
      , y :: Number
      }

What can we do with it? 🐙

derive instance eqCoordinate
  :: Eq Coordinate

derive newtype instance semiringCoordinate
  :: Semiring Coordinate

derive newtype instance ringCoordinate
  :: Ring Coordinate

derive newtype instance showCoordinate
  :: Show Coordinate

... How do I write my own code? 🐢

rotate
  :: Number -> Coordinate
  -> Coordinate

rotate angle (Coordinate { x, y })
  = Coordinate
      { x: cos angle * x - sin angle * y
      , y: sin angle * x + cos angle * y
      }

Where's the rolling circle? 🎱

rollingCirclePosition
  :: Number -> Number
  -> Coordinate

rollingCirclePosition sizeRatio time
  = rotate time initial
  where
    initial = Coordinate
      { x: 0.0
      , y: 1.0 - sizeRatio
      }

Which way up is it? ♻️

rollingCircleRotation
  :: Number -> Number
  -> Number

rollingCircleRotation sizeRatio time
  = -time / sizeRatio

Ok, but where's the pen? 🔮

penOffset
  :: Number -> Number -> Number
  -> Coordinate

penOffset sizeRatio offsetRatio rotation
  = rotate rotation
  $ Coordinate
      { x: 0.0
      , y: sizeRatio * offsetRatio
      }

Drawing 📷

How do we draw a point on the canvas? 📌

mark
  :: Configuration -> Seconds
  -> Drawing

mark { sizeRatio, offsetRatio } (Seconds time)
  = filled (fillColor colour)
  $ circle x y 2.0

Where did x and y come from? 🔭

  where
    rollingCentre
      = rollingCirclePosition sizeRatio time

    angle
      = rollingCircleRotation sizeRatio time

    Coordinate { x, y }
      = centreForCanvas
      $ rollingCentre
      + penPosition sizeRatio offsetRatio angle

... and colour? 🌈

    colour
      = hsv (time * 180.0 % 360.0) 0.8 0.8

Business logic 🏢

Dealing with the "real world" 💀

  canvas <- lift (getCanvasElementById "spirograph")
    >>= case _ of
      Just canvas -> pure canvas
      Nothing     -> throwError "No canvas :("

  lift $ setCanvasDimensions canvas
    { width: 400.0, height: 400.0 }

  context <- lift (getContext2D canvas)

Risky Live Moment 2: The finished product 🙉

Draw me like one of your French curls 🚤

  -- Current time as a stream   vvvvvvv
  stopDrawing <- lift $ animate seconds \time -> do
    let config = { sizeRatio, offsetRatio }
    render context (mark config time)

A little more maths? 👏

  let crossover
        = toNumber
        $ numerator
        $ simplify sizeRatioAsFraction

      completion
        = 2000.0 * pi * crossover

  void
    $ lift
    $ setTimeout (ceil completion) stopDrawing

Risky live moment 3: The even finisheder product 🙊

Could we have three dimensions ❓

Yes ❗

Polymorphic accessors 🔍

getX
  :: forall wrapper output anythingElse
   . Newtype wrapper { x :: output | anythingElse }
  => wrapper
  -> output

getX
  = _.x <<< unwrap

Polymorphic coordinate operations ⭐

class Coordinate (object :: Type) where
  transform
    :: (Number -> Number)
    -> (object -> object)

  fold
    :: forall m. Monoid m
    => (Number -> m)
    -> (object -> m)

Type-trickery 🚀

class GCoordinate
    (row  ::  # Type)
    (list :: RowList) where
  transform'
    :: RLProxy list -> (Number -> Number)
    -> (Record row -> Record row)

  fold'
    :: forall m. Monoid m
    => RLProxy list -> (Number -> m)
    -> (Record row -> m)

The boring case 🐌

instance gcoordinateNil
    :: GCoordinate row Nil where
  transform' _ _ = identity
  fold' _ _ _ = mempty

The interesting case 🐰

instance gcoordinateCons
    :: ( GCoordinate row tail, IsSymbol key
       , Row.Cons key Number xyz row )
    => GCoordinate row (Cons key Number tail) where
  transform' _ f record
    = modify (SProxy :: SProxy key) f
    $ transform' (RLProxy :: RLProxy tail) f record

  fold' _ f record
     = f (get (SProxy :: SProxy key) record)
    <> fold' (RLProxy :: RLProxy tail) f record

Finally... 🎊

instance coordinateImpl
    :: ( RowToList row list
       , GCoordinate row list )
    => Coordinate (Record row) where
  transform = transform' (RLProxy :: RLProxy list)
  fold      = fold'      (RLProxy :: RLProxy list)

All this for what? 🔨

offset
  :: forall row. Coordinate row
  => row -> Number

offset record
  = sqrt total
  where
    folder x = Additive (x `pow` 2.0)
    Additive total = fold folder record

Success 🎉

... Well, not quite 💔

• Floating point precision!

• (a * b ) % b === 0

• (a * 2 ) % 2 === 0

• (a * 3 ) % 3 === 0

• (a * π ) % π === 0

• (a * (π / 4)) % (π / 4) === 1.2566

• show excuse.png

• But, with a better number type, yes!

"It is left as an exercise to the reader"

Other exercises to the reader

• Stateful animation with FRP.Behavior.fixB.

• purescript-super-circles

• Continuous lines (better laptops).

• Interactive controls.

• Other types of ellipses to roll.

Summary

• Present animations on a newer laptop.

• Simple canvas drawing with purescript-drawing.

• Simple animation with purescript-behaviors.

• More examples with purescript-super-circles.

• There was life before the Internet.

Thank you!

Questions?

• 🎩 Tom Harding

• 👔 Habito (always hiring!)

• 💬 twitter.com/am_i_tom

• 🌱 github.com/i-am-tom

• 📚 tomharding.me

• 👑 github.com/jaspervdj/patat