Graphics.Web.Processing.Core.Var

Internal module defining Var type and functions. The basic interface uses these variable functions. However, they introduce a not-really-coherent behavior (see readVar).

Variables

The variable system presented here is actually an artifact to make explicit which values may change over time and which don't. It also guides the user to create variables before use them and do so in a more natural way. Somehow, it tries to be similar to IORefs, while they aren't. Strictly speaking, they do not contain any value, but it will contain a value when processing.js executes the resulting code. To make sure you are doing the right thing, variables are typed, thus forcing you to feed the correct functions with the correct values. However, this also leads to weird things like the one described in readVar.

Functions

data Var a

Type of variables.

varName :: Var a -> Text

Get the name of a variable.

newVar :: (Monad (m Preamble), ProcMonad m, ProcType a) => a -> m Preamble (Var a)

Create a new variable with a starting value. The creation of variables is restricted to the Preamble.

readVar :: (Monad (m c), ProcMonad m, ProcType a) => Var a -> m c a

Read a variable.

Funny fact: it does not matter when you execute this function. The result will always hold the last value asigned to the variable. For example, this code

 v <- newVar 10
 ten <- readVar v
 writeVar v 20
 point (10,ten)

will draw a point at (10,20).

writeVar :: (ProcMonad m, ProcType a) => Var a -> a -> m c ()

Write a value to a variable.