DDC/DestructiveUpdate
Update of base values
Values can be assigned to using the := operator.
main ()
= do x = 2
out x -- prints '2'
x := 3
out x -- prints '3'
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 are in the top level scope.
data Point
= Point { x :: Float; y :: Float; }
We'll also define an instance for the Out type-class, which we're use 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'
Accessing the x field gives us a reference to the Float object inside point.
oldX = point.x
out oldX -- prints '2.0'
When we write a literal value, this creates a whole new object.
newX = 5
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'