I've turning my Haskell OpenSCAD model types into instances of Semigroup and Monoid, but wonder if I've chosen the right "empty" value for monoids. For those not familiar with it, a monoid is a structure that has an operation (in this case, union) with an identity object, empty. The idea is that a union of anything with empty should return that same thing. Is there a convenient such object in OpenSCAD? I just used "box 0 0 0", which seems to do the right thing, but it feels like there ought to be a better option for "I need to return an object here, but don't really want to."

I'm not sure if there is a correct answer. I'm not sure if the openscad geometric operations are mathematically consistent for edge cases like passing the empty model as an argument. Mathematically, a 3D shape is a set of points, and the empty shape is the empty set. So you would expect union(X, empty) = X, intersection(X, empty) = empty. In principle, i'd expect cube(0) and group(){} to both behave as the empty shape, but in practice, they give different results when passed to intersection. cube(0) behaves as expected, while intersection(){cube(1);group(){}} returns cube(1). So I guess that "box 0 0 0" is a reasonable choice. On 14 April 2015 at 12:35, Mike Meyer <mwm@mired.org> wrote: > I've turning my Haskell OpenSCAD model types into instances of Semigroup > and Monoid, but wonder if I've chosen the right "empty" value for monoids. > > For those not familiar with it, a monoid is a structure that has an > operation (in this case, union) with an identity object, empty. The idea > is that a union of anything with empty should return that same thing. > > Is there a convenient such object in OpenSCAD? I just used "box 0 0 0", > which seems to do the right thing, but it feels like there ought to be a > better option for "I need to return an object here, but don't really want > to." > > _______________________________________________ > OpenSCAD mailing list > Discuss@lists.openscad.org > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > >