Type variable to be unified occurs in type

Question

I have a function to reconstruct a tree from 2 lists. I return a list on all branches, but I am getting an error that I don\'t understand. But I assume it has to do with the ret

Answer 1

1. Ionut gave a comprehensive explanation of how type unification works, so here is a hint:

   [preoH, recon (preoLST@preoRST, inoLST@inoRST)]
   

   The first element has type 'a and the second element has type 'a list.

   [recon (preoLST, inoLST), preoH, recon (preoRST, inoRST)]
   

   The second element has type 'a and the first and third element have type 'a list.

2. When you check lists for being empty, consider using null preoT, or handle the case using pattern matching.