Algorithm for topological sorting if cycles exist

Question

Some programming languages (like haskell) allow cyclic dependencies between modules. Since the compiler needs to know all definitions of all modules imported while compiling one

Answer 1

This is the NP-hard (and APX-hard) problem known as minimum feedback vertex set. An approximation algorithm due to Demetrescu and Finocchi (pdf, Combinatorial Algorithms for Feedback Problems in Directed Graphs (2003)") works well in practice when there are no long simple cycles, as I would expect for your application.