#Dal Segno

Write games, see your changes immediately: every time you change your code, Dal Segno rewinds your game back to the last time that piece of code was run.

demo

try it fullscreen

#Language

It's sort of like Scheme.

Run this program fullscreen

;semicolons make the rest of a line a comment
(define x 10)           ;var x = 10
(defn recur ()          ;named function definitions are global
  (color 100 200 100)   ;sets the color to be used
                        ;for future draw operations
  (fillRect 0 0 width height) ;this is the canvas
                        ;context drawing operation
  (color 0 0 230)
  (drawArc x 100 111)   ;queues a circle to be drawn
  (render)              ;actually paints queued drawings
  (set! x (  x .1))     ;change var wherever it was defined
  (if (> x 300)         ;you've got if, set!, define, defn,
      (set! x 0))       ;and lambda - that's it for special forms
  (recur))              ;no loop constructs - you have to recur!
(recur)

###Keywords

• (if cond expr1 [expr2])
• (define name expr)
• (set! name expr)
• (lambda (param...) expr1...)
• (defn name [param1...) expr1...)
• (do expr1 [expr2...])

defn expressions update a global table of named functions in addition to evaluating to a function. Use do to put multiple expressions in the body of an if, set, or define

Identifier lookup checks the local scope, outer scopes, then the standard library and builtin functions:

• standard library - written in this this language and can be stepped through
  • (map func array)
  • (reduce func array initial)
  • (filter func array)
  • (find func array)
• builtins - written in JavaScript and execute in a single tick
  • (display expr [...]) - just (console.log(args))
  • binary operators (prefix notation, like ( 2 2))
  • boolean logic
    • (and e1 e2)
    • (or e1 e2)
    • (not expr)
    • (any [expr...])
  • list operations
    • (list [e1...])
    • (get index list)
    • (first list)
    • (last list)
    • (rest list)
    • (append list expr)
    • (prepend list expr)
    • (length list)
    • (range n) - list of n numbers from 0 up to n, including 0 but not including n
    • (linspace start stop n) - list of n evenly spaced numbers from start to stop inclusive on both sides
    • (concat [list1...])
    • (zip list1)
    • (zip2 list1 list2)
    • (zip3 list1 list2 list3)
    • (zip4 list1 list2 list3 list4)
  • game math
    • (dist x1 y1 x2 y2) - Euclidian distance
    • (dist p1 p2) - Euclidian distance betwen two two-element lists
    • (randint lower upper)
    • (towards x1 y1 x2 y2) - degree heading to point
    • (towards p1 p2) - degree heading to point
    • (x_comp degrees) - float between -1 and 1
    • (y_comp degrees) - float between -1 and 1
    • (distToLine point line) - distance from point to line segment
    • (distToLine point p1 p2) - distance from point to line segment defined by p1 and p2
    • (linesIntersect line line) - whether two line segments intersect
    • (linesIntersect p1 p2 p3 p4) - whether two line segments intersect
    • (pointFromLineSegment p1 p2 p3) - distance from a point to a line segment
    • (closestPointOrLine p1 points) - the closest [x, y] point or [[x, y], [x, y]] line segment closest to the point p1 or null if no points given
    • (closestPointOrLine p1 points maxDist) - the closest [x, y] point or [[x, y], [x, y]] line segment closest to the point p1 or null if none closer than maxDist
    • (bounce x y dx dy pointOrLine) - New [dx, dy] after bouncing off of pointOrLine
    • (linesFromPoints points) - pairs of consecutive points to fom lines
  • JavaScript interop
    • (jsGet obj prop)
    • (jsSet obj prop value)
• mouseTracker
  • (mousex) - horizontal position from the left
  • (mousey) - vertical position from the top
  • (mousepos) - list of [x, y]
• drawHelpers
  • (color r g b)
  • (render) - runs queued canvas context drawing procedures
  • (drawText x y [text...])
  • (drawPoly x y list-of-x-y-pairs heading-in-degrees])
  • (drawInverseCircle x y radius)
  • (fillLine points)

If an identifier is not found in the above scopes lookup proceeds to JavaScript objects. If the found value is a function, a version of it bound to the object it was looked up on is returned. e.g. log -> console.log.bind(console)

• main lazy canvas context which is similar to a CanvasRenderingContext2d but rendering does not occur until render is called. This is where drawing functions like (fillRect x y width height) come from.
• main canvas
• console
• window

To run the code locally you'll need webpack and some loaders.

npm install
npm install -g webpack

Once those are installed, run the following:

make
python3 -m http.server 8000  # or any other static file server

and open localhost:8000 in a webbrowser.

To run the tests, install mocha and chai and run mocha on the tests:

npm install -g mocha
npm install chai
mocha src/test*

The ES5/6 features in the code are limited to those implemented in Node without requiring any flags so the tests can be run without compiling.

There are a lot of directions this could be taken in, I capped it off pretty arbitrarily because I wanted to be able to show it to people. Some particularly relevant things to do:

• Fix up DalSegno.js to allow editing and stepping in the same embed. This mostly works, but not all of the state transitions are covered.
• Move all effects to the effect canvas, currently the main lazycanvas is still doing extra work.
• An alternate interpreter mode that does not save snapshots. This would be nice for publishing programs for use.
• faster GC and fix memory leaks

I have a lot of smaller fixes I'd like to make, if you'd like to contribute let me know and I can advise on your PR. Here are a few simple fixes:

• AST highlighting regions are slightly off (usually 1 extra character on each side)
• Set focus on mouseover for Dal Segno widget for keyboard events
• Save current program in local storage instead of url
• Keep multiple programs saved at a time

There's lots more, if there's interest I can throw a lot of things up on an issue tracker.

I wrote this to play with interpreters and so I could be informed as I talked to Mary Rose Cook about her Code Lauren project.