[lower]

With x1 = Just, the program is accepted instantly by ghc. I think you need a type variable to appear twice.

[NieDzejkob]

Ah, it's still exponential, but in the size of the type, and the constants of the complexity are a bit different, so you need around x30.

[lower]

Ah, you're right. If one goes to x_{i+1}, then there will be 2^i Maybes in the type. The number of Maybe-occurrences in the type doubles in each step and sharing won't help there.