only allow non-negative quantities for various parameters?

[jvoigtlaender]

For example, radius of circle, length/width of rectangle, also the solid variants, etc.

Somewhat akin to stuff done for #49.

[jvoigtlaender]

codeworld/codeworld-api/src/CodeWorld/Picture.hs

Lines 291 to 295 in 1af6430

	thickCircle :: HasCallStack => Double -> Double -> Picture
	thickCircle a r
	| a < 0 = error "The line width must be non-negative."
	| a <= 2 * r = ThickCircle (getDebugSrcLoc callStack) a r
	| otherwise = error "The line width of a thickCircle must not be greater than its diameter."

[jvoigtlaender]

(Negative parameters are showing up in student submissions, even with negative constants, and are screwing up some detection in the tests. For negative constants, even some Hlint rules might be able to reject statically.)