diagrams/force-layout — pull request #1 by ggreif (2014-06-12 10:31:37+00:00)
Title: Make it compilable with GHC HEAD (v7.9)

See ghc bug #8883 and related discussion http://www.haskell.org/pipermail/ghc-devs/2014-June/005180.html

This is the error that GHC emits:
```
src/Physics/ForceLayout.hs:148:9:
Illegal equational constraint Scalar b ~ Scalar v
(Use GADTs or TypeFamilies to permit this)
In the context: (VectorSpace b, Scalar b ~ Scalar v)
While checking the inferred type for ?stepVel?
In an equation for ?particleStep?:
particleStep d
= stepPos . stepVel
where
stepVel p = vel .~ (d *^ (p ^. vel ^+^ p ^. force)) $ p
stepPos p = pos %~ (.+^ p ^. vel) $ p
```

Btw. as of today this message will become explained even better.