[Haskell-cafe] Are constructors strict?

I am having hard time understanding the following paragraph in "Purely functional Lazy non-deterministic programing" paper http://www.cs.rutgers.edu/~ccshan/rational/lazy-nondet.pdf The problem with the naive monadic encoding of non-determinism is that the arguments to a constructor must be deterministic. If these arguments are themselves results of non-deterministic computations, these computations must be performed completely before we can apply the constructor to build a non-deterministic result. Why does the argument to constructors must be deterministic? WHy is it that thunks are not used in this case? Thanks -- Daryoush Weblog: http://onfp.blogspot.com/