--loop disables generating multiple files

[nomeata]

I’m using Diagrams.Backend.Cairo.Cmdline.mainWith, passing an Animation B R2. Running the resulting program creates many frames. Running it with -l creates only the first one.

This is diagrams 1.2.

[byorgey]

Thanks for the report! To clarify, does running it with -l create many frames the first time, but only the first subsequent times? Or does it only generate one frame even the first time?

[bergey]

There are a couple of paths through the Cairo Backend, and the Mainable instance that handles Animation is completely unaware of looping. I'm not sure how hard it will be to add this feature.

As a stopgap, here's the bash code I use for looping:

alias ghcsandbox='ghc -global-package-db -no-user-package-db -package-db .cabal-sandbox/$(uname -m)-linux-ghc-$(ghc --numeric-version)-packages.conf.d/ -O2'
function diagramwatch {
         while true
               do inotifywait $1.hs && ghcsandbox $1 && $1
               done
}

[nomeata]

To clarify, does running it with -l create many frames the first time, but only the first subsequent times? Or does it only generate one frame even the first time?

The latter: Only one frame the first time.

[byorgey]

I think I see what's going on. The Mainable Animation instance (here: https://github.com/diagrams/diagrams-cairo/blob/master/src/Diagrams/Backend/Cairo/CmdLine.hs#L309) simply calls defaultAnimMainRender, which is defined here: https://github.com/diagrams/diagrams-lib/blob/master/src/Diagrams/Backend/CmdLine.hs#L523. That in turn calls mainRender for cairo's Mainable Diagram instance, which is what does the looping. However, it calls mainRender many times using mapM_, but the first one it calls outputs a single frame and then loops, i.e. sits there waiting for the input file to change! So control never gets returned to the mapM_ that was generating the frames.

One possible solution is to have defaultAnimMainRender turn off the looping flag for all but the last call to mainRender. That seems a little hackish though. It would also require callers to provide some sort of callback to disable looping (since defaultAnimMainRender does not know the structure of the options record being used). I'm not sure if there is a better solution.

[byorgey]

See the commit above for a potential solution; feedback welcome. Note this would require changes to diagrams-cairo as well as diagrams-rasterific (any backend which calls defaultAnimMainRender, since its type has changed).

[fryguybob]

This is somewhat related to the fix here, but I think we should get rid of the CPP in the mainable instances guarding the platforms that don't support looping. If we can't figure out how to do looping on Windows, we should just have some backup interactive loop where on a key press we recompile.