Constraint synonym for any valid back end?

[kindaro]

So I am learning diagrams for like 2 days. At first it was smooth sailing, but then I decided to
split some diagram definitions out from the Main module. This is when I met type errors like
these:

src/Draw.hs:10:74: error:
    • Couldn't match type ‘V t0’ with ‘V2’
        arising from a use of ‘circle’
      The type variable ‘t0’ is ambiguous
    • In the expression: circle 0.1
      In the expression:
        let r = (sqrt $ fromIntegral $ Graph.size graph :: Double)
        in circle 0.1
      In an equation for ‘node’:
          node
            = let r = (sqrt $ fromIntegral $ Graph.size graph :: Double)
              in circle 0.1
    • Relevant bindings include node :: t0 (bound at src/Draw.hs:10:5)

 

src/Draw.hs:1:1: error:
    solveWanteds: too many iterations (limit = 4)
      Unsolved: WC {wc_simple =
                      [W] $dTrailLike_a27Vn {0}:: TrailLike
                                                    (QDiagram backEnd s0 s1 Any) (CDictCan)
                      [D] _ {2}:: base-4.12.0.0:Data.Typeable.Internal.Typeable
                                    s0 (CDictCan)
                      [D] _ {2}:: RealFloat s0 (CDictCan(psc))
                      [D] _ {1}:: Renderable (Path V2 s0) backEnd (CDictCan(psc))
                      [WD] $dFloating_a27Vo {2}:: Floating s0 (CDictCan)
                      [WD] $dOrd_a27Vp {2}:: Ord s0 (CDictCan)
                      [W] $dMetric_a27Vq {1}:: Metric s0 (CDictCan)
                      [WD] $dFractional_a27Vr {0}:: Fractional s0 (CDictCan)
                      [D] _ {0}:: Num s0 (CDictCan)
                      [D] _ {2}:: Eq s0 (CDictCan)
                      [D] _ {2}:: RealFrac s0 (CDictCan)
                      [D] _ {2}:: Real s0 (CDictCan)
                      [WD] hole{co_a27Vk} {8}:: s0 ~ V2 (CNonCanonical)}
      Set limit with -fconstraint-solver-iterations=n; n=0 for no limit

 

I would probably be struggling with them for a lot longer, have I not found this awesome Stack
Overflow answer from Daniel @bergey. In short, there is defined a type synonym like this:

type Back b = (V b ~ V2, N b ~ Double,
    Renderable (Path V2 Double) b, Renderable (Text Double) b)

dia2 :: Back b => Diagram b
dia2 = circle 1 # fc blue

I tried it and it actually solves all those type errors!

I understand that this type synonym only covers 2D back ends. But is it not nevertheless awesome?

I propose that type synonyms like this are included in Diagrams.Prelude and written about in the
awesome official manual.