Perplexing GHC-7.0.3 behavior with hairy type-level code (regression from 6.12.3??)

[Simon Peyton-Jones]

It's hard to say much without a particular program to look at.  But when type families or functional dependencies are involved you can certainly get situations where
	f :: <ty>
but if you write

	g ::<ty>
	g = f

the program is rejected.  Sounds similar to what you are seeing.  Look here:

http://www.haskell.org/haskellwiki/GHC/Type_families#Injectivity.2C_type_inference.2C_and_ambiguity

If you are still puzzled, by all means try to boil out a test case, the smaller the better.   A bug is far from impossible.

Simon

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Bjorn Buckwalter
|  Sent: 22 April 2011 16:19
|  To: GHC Users
|  Subject: Perplexing GHC-7.0.3 behavior with hairy type-level code (regression from
|  6.12.3??)
|  
|  Hello all,
|  
|  I am doing some fairly hairy type-level stuff with
|  FunctionalDependencies, UndecidableInstances, and other. Without going
|  into details I have the following function which compiles fine:
|  
|    vecMat :: (Transpose m m', MatrixVector m' v v', Num a)
|           => Vec v a -> Mat m a -> Vec v' a
|    vecMat v m = transpose m `matVec` v
|  
|  
|  However, I am perplexed by the following:
|  
|  1. This does NOT compile despite the type signature being identical to
|  that of 'vecMat':
|  
|    vecMat2 :: (Transpose m m', MatrixVector m' v v', Num a)
|            => Vec v a -> Mat m a -> Vec v' a
|    vecMat2 v m = vecMat v m
|  
|  The error message is along the lines of the below where 'HMap ...' is
|  the context for the MatrixVector instance... I'll provide more details
|  as needed.
|  
|      Could not deduce (HMap (DotProd, v) m' v')
|        arising from a use of `vecMat'
|      from the context (Transpose m m', MatrixVector m' v v', Num a)
|        bound by the type signature for
|                   vecMat2 :: (Transpose m m', MatrixVector m' v v', Num a) =>
|                              Vec v a -> Mat m a -> Vec v' a
|  
|  
|  2. If I omit the type signature the definition of 'vecMat2' does compile:
|  
|    vecMat2 v m = vecMat v m
|  
|  3. If I omit the second argument it also compiles:
|  
|    vecMat2 v = vecMat v
|  
|  4. However, if I omit both arguments it does NOT compile:
|  
|    vecMat2 = vecMat
|  
|  
|  So I guess I have two questions. First: why would 1 not compile?
|  Second: why are 2, 3, and 4 not equivalent? Pointers to relevant
|  documentation welcome.
|  
|  I broke my GHC-6.12.3 installation when upgrading to the latest HP
|  with GHC-7.0.3 so I cannot test 2, 3, and 4 but I know that 1 DID
|  compile on GHC-6.12.3. Is the change of behavior in GHC-7.0.3 a bug or
|  a bug fix?
|  
|  I'll be happy to elaborate on the code if it would be useful, and try
|  to find a minimal example. But I wanted to check if the behavior I am
|  seeing makes any sense at all first. In case you cannot wait the
|  'matVec' definition is from:
|  
|  https://github.com/bjornbm/dimensional-
|  vectors/blob/master/Numeric/Units/Dimensional/LinearAlgebra/Matrix.hs
|  
|  
|  Thanks,
|  Bjorn
|  
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