This is an internal module, useful if you want to implemented your own FRP library. If you just want to use FRP in your project, have a look at Reactive.Banana instead.

The Build monad is an instance of MonadFix and supports value recursion. However, it is built on top of the IO monad, so the recursion is somewhat limited.

The main rule for value recursion in the IO monad is that the action to be performed must be known in advance. For instance, the following snippet will not work, because putStrLn cannot complete its action without inspecting x, which is not defined until later.

  mdo
      putStrLn x
      let x = "Hello recursion"

On the other hand, whenever the sequence of IO actions can be known before inspecting any later arguments, the recursion works. For instance the snippet

  mdo
      p1 <- mapP p2
      p2 <- neverP
      return p1

works because mapP does not inspect its argument. In other words, a call p1 <- mapP undefined would perform the same sequence of IO actions. (Internally, it essentially calls newIORef.)

With this issue in mind, almost all operations that build Latch and Pulse values have been carefully implemented to not inspect their arguments. In conjunction with the Cached mechanism for observable sharing, this allows us to build combinators that can be used recursively. One notable exception is the readLatch function, which must inspect its argument in order to be able to read its value.