haskell - How can I branch on the value inside a Reflex Dynamic? -

in simplest case, have dynamic t bool, , when value true, want single empty div exist, , when value false, don't want there dom element.

slightly more generally, if have dynamic t (either mya myb), , have functions know how render given dynamic t mya or dynamic t myb, how call appropriate function render?

if need switch widget need 1 of:

dyn :: monadwidget t m => dynamic t (m a) -> m (event t a) source 

or

widgethold :: monadwidget t m => m -> event t (m a) -> m (dynamic t a) 

since you've mentioned you've got dynamic @ hand, we're gonna use dyn:

app =   switched <- button "alternate!"   flag <- folddyn ($) false (not <$ switched) -- have dynamic t bool   w <- mapdyn mywidget flag -- in latest reflex can 'fmap'   dyn w   return ()  mywidget :: monadwidget t m => bool -> m () mywidget false = blank mywidget true = el "div" $ blank 

the basic rule that, due higer-order nature of reflex, if want swap-out something, need have event/dynamic yields widget value. that's why dyn takes dynamic t (m a) parameter (and appropriately, widgethold takes event t (m a). , that's why we've mapped on dynamic t bool have dynamic has our widget building action value.

it's worth mentioning, neither dynamic/widgethold virtual dom/diffing speed rendering. reflex can more explicit updates (a dynamic/event t text can affect node text directly, without rerendering whole node) , should take advantage of that. if not large portions of page swapped , can yield significant performance hit.