Alain Robbe-Grillet declared that his movie was inspired by Bioy’s novel, but it isn’t simply an adaption of Bioy’s work into a film. Upon viewing the film, there is clearly a lot of thematic, and even plot-based, overlap between the two, but each is also clearly independent from the other.
In this way, I think Robbe-Grillet made a movie "based on a novel" in the sense that Viktor Shklovsky would have wanted to see it happen. In his long essay/short book Literature and Cinematography, Shklovsky decries the many book-to-film adaptations already available in the 1920s as being simply the plot of the book rendered on the screen.
What Shklovsky would have preferred to see were movies that explored the cinema’s unique capabilities for telling a story; what he got was Dickens acted out and filmed, more or less faithfully following the text.
Last Year at Marienbad is a story that I think could only be told cinematically. In Robbe-Grillet’s juxtaposition of certain scenes and images (jumping back and forth to suggest relationships, without ever making it precisely clear what he is jumping between); in his voiceovers that seem to narrate events being depicted on-screen even as we wonder what is the exactly relationship between each, and who is talking to whom; in these devices and others, I think Robbe-Grillet has made something that could not precisely, or even grossly, be recreated in another medium.
This much we know: in both Bioy and Robbe-Grillet there is a man who dearly wants to communicate with a woman; in both he is doomed to fail, but, perhaps through his failures achieve a kind of communication that one might say is the best any of us could hope for when trying to communicate with another person. The circumstances of the book and film, however, are vastly different.
So too are their styles. Although Bioy’s novel is surreal and satisfyingly innovative, he tells us a more or less straightforward story through the frame of a journal. Robbe-Grillet gives us an agglomeration of images that are fundamentally impenetrable as a narrative; we can make guesses as to the story that might be told from what we see on the screen, but there is no way any viewer can claim to have found the definitive narrative in the movie.
In a strange sort of way, the two deepen the experience of each without closing off any avenues. In my experience, there are points of intersection between the book and the movie, images, devices, dialog that could conceivable work well in both. I found these intersections to be like aids that encouraged me to consider both the book and the movie in new ways. But never did I feel like one of these clues had closed off a reading that I had previously entertained.