Lvalues
In Skip, ! designates which part of the lvalue is going to be modified. That notion is pretty unconventional, so let's just run through a few examples:
x.!field1 = 0
That last example works exactly like in most programming languages; the object that the variable x points to is going to be modified, and the slot that corresponds to field1 is going to be assigned the value 0. Said differently, if x was pointing to MyObject{ field1 => 42 } it will point to the object MyObject{ field1 => 0 } after this expression is evaluated, and the modification will happen by physically updating the field field1.
!x.field1 = 0
In this case, the object that the variable x points to is going to be left unmodified. Instead, the variable x is going to be updated to point to a new copy of the object pointed to by x where field1 has the value 0. Said differently, if x was pointing to MyObject{ field1 => 42 } it will still point to an object of type MyObject{ field1 => 0 } after the expression is evaluated (like in the previous example), but by physically updating the local x this time. This notation is a bit disturbing at first, but you will learn to love it when manipulating nested immutable values, which is very common in Skip.
Let's take a few more examples.
x.!field1.field2 = 0
That is a mix of the two behaviors, x.field1 is going to be physically modified to point to a copy of the object that was there, with the field field2 set to the value 0.
(!this, y) = this.f(...);
= is both used for assignments and variable introduction for a reason. It can be useful for an lvalue to both introduce new variables and modify existing ones. For example, when dealing with immutable objects, it will be often useful to update the variable this to point to a new version of the object.