Report Dynamic-based causality loops better

[ryantrinkle]

When a causality loop is created by joining a Dynamic whose contents depend on itself, reflex reports this as

<interactive>: heightBagRemove: Height 1 not present in bag HeightBag {_heightBag_size = 2, _heightBag_contents = fromList [(0,1)]}
CallStack (from HasCallStack):
  error, called at src/Reflex/Spider/Internal.hs:1015:14 in reflex-0.5.0-4m111lPPOvPBxxVtye35C3:Reflex.Spider.Internal

Ideally, it should report this as a causality loop explicitly, instead.

Here's some code that exhibits the problem:

{-# LANGUAGE RecursiveDo #-}

import Control.Monad
import Reflex.Dom

main = mainWidget $ do
  postBuild <- getPostBuild
  rec dd <- holdDyn (constDyn ()) (d <$ postBuild)
      let d = join dd
  performEvent_ $ return <$> updated d

[ryantrinkle]

That's a good old traditional Haskell loop - no FRP involved. 'sample b' is being used to initialize 'dyn', which is where 'b' comes from. Pretty nearly the same as `rec r <- newIORef =<< readIORef r`

…

On Thu, Aug 30, 2018 at 7:44 PM Kosyrev Serge ***@***.***> wrote: This makes me wonder -- what would be the semantics of the following? loopDyn = mdo let b = current dyn e ← delay 0 $ updated dyn dyn ← buildDynamic (sample b) $ tag b e pure dyn I found that it hangs my (reflex-glfw-based) Reflex host, preventing it from processing events -- but doesn't cause a cpu busyloop. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#120 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABGlYJCtMBBhhh31Y5Y_EnHLxHJlAWj0ks5uWHjYgaJpZM4OQUM0> .