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_inf... 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@haskell.org [mailto:glasgow-haskell-users- | bounces@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 | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users