Like the 1932 movie Freaks? (spoilers, let's say)
From a high level they both involve a main character who turns into something they see (and are 'fascinated' by) at the beginning of the movie. And of course the obvious setting in a carnival.
shareKind of the opposite of "Freaks", in that in "Freaks" we see life through the eyes of the social outcasts who work at the carnivals.
"Nightmare Alley" is an Icarus story, the story of someone's rise and fall, and to him, the freaks at the carnival are the stuff of living nightmares rather than the human beings Todd Browning saw.