Return values ​​and assertions from functions

I am curious about the difference in behavior between using a statement return

and defining a function with a templatefoo() = Expression

It is my understanding from Funtions that a function body can be = Expression

and return

accepts either nothing orExpression

The reason I am asking is because, in my understanding of the documentation, the following should not work:

map[int,int] foo(map[int,int] env) = env[0] = 1;

      

        
        
        
      

    

Which it doesn't do (causing a parsing error) since the assignment is Statement

. However, if I modify this function a bit:

map[int,int] foo(map[int,int] env) { return env[0] = 1; }

      

        
        
        
      

    

It seems to work. I am confused because although I return

have to accept expressions. Also, it looks like when playing with the REPL this assignment returns Value

, so it would be nice to define functions that assign and return the modified map.

Maybe I missed something.