If I am not wrong, FP-people say that absence of side effects simplify proofs of correctness (wrt specifications), not that proofs of programs with side effects are impossible.

For a comparison of 4 methods of proving programs with side effects (Floyd–Hoare logic, E. W. Dijkstra's Discipline of Programming, Cliff B. Jones' Vienna Development Method (VDM) and E. C. R. Hehner's Practical Theory of Programming) see:

E.C.R.Hehner, What's wrong with formal programming methods? (PDF), Conference on Computing and Information, Ottawa, 1991 May (keynote address, invited); chapter in Advances in Computing and Information, Lecture Notes in Computer Science v.497, Springer, Berlin, 1991, p.2-23

In reply to Provability possible for PL with side effects by Anonymous Monk
in thread Is it worth using Monads in Perl ? and what the Monads are ? by rootcho

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