[yorwba]

Those two programs don't use a in the Nothing case, they have Just _ instead, i.e. a hole. Those are just as valid as any other program with a hole. You could fill the hole with undefined if you wanted to.

[Twisol]

The article was edited in the interim. The error the ancestor posts describes did exist before (I saw it, too), but no longer.

[dllthomas]

As noted by Twisol, those holes previously contained `(fab a)`.

I'd delete my comment now that it's addressed, but 1) it's too late, and 2) I'm too happy about "unbound usings".