Functional dependencies vs. type families
(type functions in foreign declarations)
Latest revision as of 20:35, 27 January 2014
When I reported a typechecker performance problem related to functional dependencies I promised to try to convert from functional dependencies to type families.
Thus I converted my code and the llvm package to type-families:
Here are some of my experiences:
 1 Advantages of TypeFamilies
 1.1 Speed
For what I did the type families solution was considerably faster than the functional dependencies code at least in GHC-7.4.1. Thus the bug in ticket 5970 does no longer hurt me. (In GHC-6.12.3 the conversion to type families made the compilation even slower.)
 1.2 Anonymous type function values
One of the most annoying type classes of the llvm package was the IsSized class:
class (LLVM.IsType a, IsPositive size) => IsSized a size | a -> size
where size is a type-level decimal natural number.
Many llvm functions require that an LLVM type has a size where the particular size is not important. However, I always have to name the size type. I also cannot get rid of it using a subclass, like
class (IsSized a size) => IsAnonymouslySized a where
The conversion of this type class to type families is straightforward:
class (IsType a, PositiveT (SizeOf a)) => IsSized a where type SizeOf a :: *
I can also easily define sub-classes like
class (IsSized a) => C a where
 1.3 Type functions in foreign declarations
A foreign import or export must not have type constraints but it can contain type functions. That is, you must not declare
foreign import ccall unsafe "llvm_convert" convert :: (MakeValueTuple a b) => StablePtr a -> IO (Ptr b)
but you can declare
foreign import ccall unsafe "llvm_convert" convert :: StablePtr a -> IO (Ptr (ValueTuple b))
 1.4 No TypeSynonymInstancesAt the right hand side of a
type instance F T = String
This feels somehow more correct than refering to a type synonym in a class instance head like in
instance C T String where
 1.5 No FlexibleInstances
The same applies to
instance C (T a) (A (B a))
which is a flexible instance that is not required for
type instance F (T a) = A (B a)
 1.6 No MultiParamTypeClass, No UndecidableInstances
I have some type classes that convert a type to another type and a tuple of types to another tuple of types where the element types are converted accordingly. With functional dependencies:
class MakeValueTuple haskellTuple llvmTuple | haskellTuple -> llvmTuple where instance (MakeValueTuple ha la, MakeValueTuple hb lb) => MakeValueTuple (ha,hb) (la,lb)
The class is a multi-parameter type class and the instance is undecidable.
This is much simpler with type families:
class MakeValueTuple haskellTuple where type ValueTuple haskellTuple :: * instance (MakeValueTuple ha, MakeValueTuple hb) => MakeValueTuple (ha,hb) where type ValueTuple (ha,hb) = (ValueTuple ha, ValueTuple hb)
Thus summarized: Type families may replace several other type extensions. If I ignore the associated type functions then many classes become Haskell 98 with Haskell 98 instances. This is good because those instances prevent instance conflicts with other non-orphan instances.
 2 Disadvantage of TypeFamilies
 2.1 Redundant instance arguments
I have to write the type arguments both in the instance head and in the function argument. This is especially annoying in the presence of multi-parameter type classes with bidirectional dependencies. E.g.
class (a ~ Input parameter b, b ~ Output parameter a) => C parameter a b where type Input parameter b :: * type Output parameter a :: * process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where type Input (FilterParam a) (FilterResult v) = v type Output (FilterParam a) v = FilterResult v
With functional dependencies it was:
class C parameter a b | parameter a -> b, parameter b -> a where process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where
 2.2 Bidirectional dependencies
In GHC-6.12.3 it was not possible to write
class (a ~ Back b, b ~ Forth a) => C a b where
Fortunately, this is now allowed in GHC-7. But bidirectional dependencies are still cumbersome to work with as shown in the example above.
 2.3 Equality constraints are not supported for newtype deriving
Not so important, just for completeness:
 3 Confusions
 3.1 Upper case type function names
Why are type function names upper case, not lower case? They are not constructors after all. Maybe this is one reason, why I forget from time to time that type functions are not injective.
Sure, lower-case type variables are implicitly forall quantified in Haskell 98. In the presence of lower-case type functions we would need explicit forall quantification.
 3.2 Why can associated types not be exported by C(AssocType) syntax?
Why must they be exported independently from the associated class?
 3.3 FlexibleContextsThe context
 4 See also
- The llvm package converted to type families: http://code.haskell.org/~thielema/llvm-tf/
- Haskell Cafe post
 5 Footnotes
This article was written in June 2012 when GHC-7.4.1 was the current version of GHC.