如何在F＃上运行IO操作

Question

There were IO monad on haskell to keep it pure. I know f#/ocaml supports mutable state but how IO operations works actually. What is corresponding concept to haskell's IO monad.

Answer 1

F# is not pure. It inherits the Caml tradition of allowing IO and state changes (eg assignment to mutable variables) everywhere. For instance:

let f : int -> int =
   fun x -> 
      printf "Hello\n";
      x+1

printf "%d\n" (f 1 + f 2)      (* prints Hello twice, then 5 *)

Monads can still be found in F#, though, if you want to use them. Computation expressions correspond to monads, roughly.