DDC/DestructiveUpdate
< DDC
1 Update of base values
Values can be updated with the (:=) operator, which copies its second argument over its first.
main () = do x = 2 out x -- prints '2' x := 3 out x -- prints '3'
2 Update of algebraic data
The structure of algebraic data can be modified with the (#=) operator. This replaces the constructor at a particular node with a new one.
We'll use a simple point type as an example. In this definition, x and y are field names and are local to the Point type. This is different from Haskell, where all field names become functions in the top level scope.
data Point = Point { x :: Float; y :: Float; }
We'll also define an instance of the Out type-class, which we're using to print things to stdout until the implementation of dictionary passing for type classes is finished.
instance Out Point where out (Point x y) = println $ parens (show x % "," % show y)
Values of Point type are constructed in the standard way, and we can use the field names to access their components.
main () = do point = Point 2.0 3.0 -- construct a new point out point -- prints '(2.0, 3.0)' out point.x -- prints '2.0'
Projecting the x field gives us a handle to the Float object inside point.
oldX = point.x
out oldX -- prints '2.0'
Binding a literal value allocates a new object.
newX = 5.0
We can now make point reference this new object.
point#x #= newX
out point -- prints '(5.0, 3.0)'
out oldX -- oldX is still '2.0'
3 Reference projections
In Disciple, we don't need to change our data definitions to include Ref or IORef types before we can update them. References are created on the fly with the reference projection operator (#).
In the previous example, when we used (#) a reference was created which held a pointer into the point object.
(point # x) :: Ref Float
The (#=) operator is just a regular function which has the (simplified) type:
(#=) :: forall a. Ref a -> a -> ()
When we then evaluated:
point#x #= newX
The pointer inside point which used to point to oldX was updated to point to newX.