Sequence points in C and CPS/ANF compilation in Lisp are kind of related to this. Both monads and applicatives enable you to define f(g(), h()), where f, g, and h are stateful computations of some sort, but monads force you to specify which of g and h is invoked first [so it’s more like x = g(), y = h(), f(x, y)] while with applicatives the implementor of the stateful model can decide what happens in such cases.
[Disclaimer: I don’t know how QuickCheck actually works, so I’d appreciate a sanity check (hah) of the following from somebody with actual knowledge on the matter.]
GP’s point, if I understood it correctly, is as follows. If you’re doing randomized testing, then g and h could perhaps be defined as { for (i=0; i<n; i++) { int x = rand(); if (!fork()) return x; } } (and yes, in monad-land the moral equivalent of fork() is admissible as what I vaguely called a “stateful computation”). You see how strict ordering of side effects enforced by monads essentially forces you into a depth-first search of the state space (although I guess you could do iterative deepening?), while applicatives can allow for different exploration strategies (ones more interesting than C’s official behaviour of “that’s UB, you fool”).
C sequence points are arbitrarily imposed by imperial decree in the standard; they are not linked to any evaluation causality.
Dependency-driven de facto evaluation orders have always existed in C though: things that you can figure out must be ordered even though there isn't a sequence point.
The standard spells out that expressions like i = i + i are okay, but actually it's not necessary to do so. The assignment cannot take place until the assigned value is known. The assigned value is not known until the + is evaluated and the + cannot be evaluated until the operand values are retrieved. Therefore, the retrieval of the prior value of i necessarily precedes the movement of the new value into i.
This, rather than sequence points, is rather a bit like monadic sequencing.
> The assignment cannot take place until the assigned value is known.
Of course it can. It's retrieving the value out of the lvalue is what cannot be done and even then you can still stretch this delay further with the "as-if" rule. Haskell does something similar with its lazy evaluation IIRC.
> C sequence points are arbitrarily imposed [and] not linked to any evaluation causality.
That’s exactly my point. Writing g >>= \x -> h >>= \y -> f x y is like writing (x = g(), y = h(), f(x,y)), with an arbitrary ordering between computations g and h that the programmer is forced to choose, whereas f <$> g <*> h is like f(g(), h()), with no ordering between g and h imposed by the applicative model itself, similar to the absence of a sequence point between sibling arguments in C.
[Disclaimer: I don’t know how QuickCheck actually works, so I’d appreciate a sanity check (hah) of the following from somebody with actual knowledge on the matter.]
GP’s point, if I understood it correctly, is as follows. If you’re doing randomized testing, then g and h could perhaps be defined as { for (i=0; i<n; i++) { int x = rand(); if (!fork()) return x; } } (and yes, in monad-land the moral equivalent of fork() is admissible as what I vaguely called a “stateful computation”). You see how strict ordering of side effects enforced by monads essentially forces you into a depth-first search of the state space (although I guess you could do iterative deepening?), while applicatives can allow for different exploration strategies (ones more interesting than C’s official behaviour of “that’s UB, you fool”).