Is spoon unsafe in Haskell?

Question

So there\'s a library in Haskell called spoon which lets me do this

safeHead :: [a] -> Maybe a
safeHead = spoon . head

but it also lets

Answer 1

There is one tricky point where, if you use it, doing what seems like an innocent refactor can change the behavior of a program. Without any bells and whistles, it is this:

f h x = h x
isJust (spoon (f undefined)) --> True

but doing perhaps the most common haskell transformation in the book, eta contraction, to f, gives

f h = h
isJust (spoon (f undefined)) --> False

Eta contraction is already not semantics preserving because of the existence of seq; but without spoon eta contraction can only change a terminating program into an error; with spoon eta contraction can change a terminating program into a different terminating program.

Formally, the way spoon is unsafe is that it is non-monotone on domains (and hence so can be functions defined in terms of it); whereas without spoon every function is monotone. So technically you lose that useful property of formal reasoning.

Coming up with a real-life example of when this would be important is left as an exercise for the reader (read: I think it is very unlikely to matter in real life -- unless you start abusing it; e.g. using undefined the way Java programmers use null)