What is the order of evaluation in F#?

Question

I was reading this and now wonder: what is the evaluation order in F#?

Obviously ; makes effects happen in a sequential fashion. But what about things like function calls or applications, order of evaluation for operators, and the like.

I've glanced at the F# spec, but there is no mention of that. Thanks for any insight!

Answer 1

I found some emails where we fixed the implementation to have a rigid application order. The code

open System

let f a =
   Console.WriteLine "app1"; 
   fun b -> 
       Console.WriteLine "app2"; 
       ()

(Console.WriteLine "f"; f) (Console.WriteLine "arg1") (Console.WriteLine "arg2")

will print "f", "arg1", "arg2", "app1", "app2". However this didn't make it into the spec. I'll file a spec bug.

(Some other portions of the spec are already more explicit, e.g.

---

6.9.6 Evaluating Method Applications

For elaborated applications of methods, the elaborated form of the expression will be either expr.M(args) or M(args).

• The (optional) expr and args are evaluated in left-to-right order and the body of the member is evaluated in an environment with formal parameters that are mapped to corresponding argument values.

• If expr evaluates to null then NullReferenceException is raised.

• If the method is a virtual dispatch slot (that is, a method that is declared abstract) then the body of the member is chosen according to the dispatch maps of the value of expr.

---

That said, some experts believe that you will live a longer, happier life if you do not rely on evaluation order. :) )

(Possibly see also

http://blogs.msdn.com/ericlippert/archive/2009/11/19/always-write-a-spec-part-one.aspx

http://blogs.msdn.com/ericlippert/archive/2009/11/23/always-write-a-spec-part-two.aspx

for more on how easy it is to screw things up with evaluation order.)