arrowFromLocatedTrail function

[pnutus]

I want to annotate angles by using arcs with arrowheads on them. I would like to say

arrowFromLocatedTrail (arc fromAngle toAngle)

but there didn't seem to be a function that turned a trail into an arrow. So I wrote that function:

{-# LANGUAGE FlexibleContexts #-}

arrowFromLocatedTrail :: Renderable (Path R2) b => Located (Trail R2) -> Diagram b R2
arrowFromLocatedTrail = arrowFromLocatedTrail' with

arrowFromLocatedTrail' :: Renderable (Path R2) b 
                       => ArrowOpts -> Located (Trail R2) 
                       -> Diagram b R2
arrowFromLocatedTrail' opts trail = arrowBetween' opts' start end
  where opts' = opts & arrowShaft .~ unLoc trail
        start = atStart trail
        end   = atEnd trail

I think it might be useful to include in Diagrams.TwoD.Arrow.

[byorgey]

Thanks, this looks useful indeed. Would you be willing to open a pull request? See here for some guidance if you are not sure how:

http://www.haskell.org/haskellwiki/Diagrams/Contributing

[pnutus]

There is an issue with closed trails – they turn into no Arrow at all. It seems like Arrows starting and ending in the same place disappear (# scale 0?). Related to #151, perhaps?