[kiiski]

The basic idea is to have a uniform way of setting values to variables or calling a setter method. That's very helpful for writing macros that need to both read and set a value.

For example, the `+=`-operator found in many languages is implemented as a macro in CL (INCF). It needs to read the value of a place, increment it and then set the new value to the same place.

Doing that with a variable is easy. Using non-lisp syntax

  x += 5 

expands to

  x = x + 5

Doing that with object slots is easy too, if you don't mind accessing the slot directly. However, if you want to have getter and setter methods (which may not even correspond directly to a slot in the object) the expansion would look different.

  o.setFoo(o.getFoo() + 5)

or if the getter and setter can have the same name

  o.foo(o.foo() + 5)

Since the expansion is different, the macro would have to figure out whether it's dealing with a variable or an accessor method and behave differently. With generalized references you can use the accessor method as if it was a variable

  o.foo()      // calls the getter
  o.foo() = 5  // calls the setter

Now you easily can expand

  o.foo() += 5

to

  o.foo() = o.foo() + 5

just the same as you would with a variable. Behind the scenes you still have separate getter and setter methods, but the macro doesn't need to worry about that.

[tel]

I feel like functional lenses have vastly improved upon this idea now by (a) being first class, (b) being completely composable, and (c) being general enough to include notions of access into sum types and having multiple targets.