Proposal: Rework polygons and lines

[Chris Smith]

Hey everyone!

I mentioned a few weeks ago that we're working on putting together a
more complete educational curriculum using CodeWorld. In that
process, I've come to think we might need to make some changes that
will, again, break a lot of existing code. Sorry!

Here are the changes I have in mind. Any comments are welcome.

Currently, we have three functions:

line :: [Point] -> Picture
thickLine :: ([Point], Number) -> Picture
polygon :: [Point] -> Picture

Here, "line" is a connected sequence of line segments. "thickLine" is
the same but with thickness, and "polygon" is a filled polygon.

Here's what I propose instead:

-- A line segment.
line :: (Point, Point) -> Picture

-- A line segment with thickness.
thickLine :: (Point, Point, Number) -> Picture

-- An open polygon.
polygon :: [Point] -> Picture

-- An open polygon with a thick border.
thickPolygon :: ([Point], Number) -> Picture

-- A filled polygon.
solidPolygon :: [Point] -> Picture

If there's a strong need, I could also add:

-- Connected sequence of lines.
path :: [Point] -> Picture

-- Connected sequence of lines, with thickness.
thickPath :: ([Point], Number) -> Picture

But honestly, I tend to think these are special-purpose enough that
students who want them could write them using pattern matching on
lists. For example:

path(p:q:more) = line(p, q) & path(q:more)
path(other) = blank

and similarly for thickPath. Or with list comprehensions:

path(points) = pictures[ line(p,q) | p <- points | q <- rest(points) ]

Thoughts?

[Chris Smith]

A less extreme version of this would be to leave line and thickLine as
is, but change polygon to draw an open polygon, and add thickPolygon
and solidPolygon.

[Bartosz Wójcik]

I've just started looking at this project/tool with intention of introducing programming to my 11 years old son. I've found awkward that line is of type line :: [Point] -> Picture. So for me the proposal is natural.
BTW: first impression is very positive - it's cool.
Best,
Bartosz

[Chris Smith]

Last call for comments on this.

If I don't hear any strong objections soon, then the changes I'll be making are:

1. Rename the function currently called polygon to solidPolygon.
2. Define polygon to draw an open polygon. In other words, define
polygon(points) = line(points ++ take(points, 1))
3. Add an analogous thickPolygon(points, k) = thickLine(points ++
take(points, 1), k)

This will be a breaking change, because polygon has been renamed. At
this time, I don't plan to make the proposed changes to line or
thickLine; they will still take lists as parameters. Those changes
are still on the table, but they don't seem like quite the slam-dunk
improvement that we get from making polygon follow the same pattern as
other shapes.

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[Chris Smith]

(As an aside, I've already made a change so that when the first and
last parameters to line or thickLine are the same, I close the shape.
This was necessary to fix thickRectangle, among other things. I
considered that a bug. Once this change is finished, an argument
could be made that I should revert the change to line and thickLine,
and only close paths with polygon and thickPolygon.)

[Chris Smith]

This change is live. You'll need to rename old uses of polygon to
solidPolygon. I also reverted the change to thickLine, so if you try
to draw a closed shape using thickLine, the ends won't stitch
together. Draw your closed shapes using thickPolygon instead.