Poor indication of type error location

[guest]

Using {-# OPTIONS_GHC -XArrows -fno-monomorphism-restriction #-}, and Yampa Starting from line 32, my program contains:

fireworkSF :: Point2 GL.GLdouble -> Point2 GL.GLdouble -> Object
fireworkSF p0 pFinal = proc _ -> do
  rec
    position <- (p0 .+^) ^<< integral -< v0
    let shouldExplode = position == pFinal
    let killReq = if shouldExplode then Event () else noEvent
    let spawnReq = if shouldExplode
                     then Event (Explode pFinal)
                     else noEvent
  returnA -< ObjectOutput {oState = FireworkState position
                          ,oKillReq = killReq
                          ,oSpawnReq = spawnReq}
  where
    v0 = (pFinal ^-^ p0) ^/ 2

The type error reports:

Firework.hs:32:0:
    Couldn't match expected type `Point2 GL.GLdouble'
           against inferred type `Vector2 GL.GLdouble'
    When using functional dependencies to combine
      AffineSpace (Point2 a) (Vector2 a) a,
        arising from the instance declaration at <no location info>
      AffineSpace (Point2 GL.GLdouble) (Point2 GL.GLdouble) a,
        arising from a use of `.+^' at Firework.hs:34:16-23
    When generalising the type(s) for `fireworkSF'

Indicating that the bug is something to do with line 34. The actual bug is that the last line of the paste should read:

    v0 = (pFinal .-. p0) ^/ 2

[guest]

Sorry, here's the code again:

fireworkSF :: Point2 GL.GLdouble -> Point2 GL.GLdouble -> Object
fireworkSF p0 pFinal = proc _ -> do
  rec
    position <- (p0 .+^) ^<< integral -< v0
    let shouldExplode = position == pFinal
    let killReq = if shouldExplode then Event () else noEvent
    let spawnReq = if shouldExplode
                     then Event (Explode pFinal)
                     else noEvent
  returnA -< ObjectOutput {oState = FireworkState position
                          ,oKillReq = killReq
                          ,oSpawnReq = spawnReq}
  where
    v0 = (pFinal ^-^ p0) ^/ 2

And the error:

Firework.hs:32:0:
    Couldn't match expected type `Point2 GL.GLdouble'
           against inferred type `Vector2 GL.GLdouble'
    When using functional dependencies to combine
      AffineSpace (Point2 a) (Vector2 a) a,
        arising from the instance declaration at <no location info>
      AffineSpace (Point2 GL.GLdouble) (Point2 GL.GLdouble) a,
        arising from a use of `.+^' at Firework.hs:34:16-23
    When generalising the type(s) for `fireworkSF'

and the correct line: v0 = (pFinal .-. p0) ^/

[simonpj]

I can't say I understand just what's going on here. It's difficult to fix this unless we can reproduce it, but you don't give enough information to do that. If you have enough time to upload a reproducible test case, that'd be great.

Simon

[igloo]

No response from submitter, so closing.