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Functional Dependencies
Brief Explanation
Functional dependencies (borrowed from relational databases) are a partial solution to the ambiguities that plague multiparameter type classes, e.g.
class ... => C a b c  a > b
says that any two instances of C that agree in the first parameter must also agree in the second.
References
 Type Classes with Functional Dependencies by Mark P. Jones, in ESOP 2000. A semiformal description of a more restricted system than implemented by GHC and Hugs.
 Understanding functional dependencies via Constraint Handling Rules by Martin Sulzmann, Gregory J. Duck, Simon Peyton Jones, and Peter J. Stuckey, September 2005. This paper explains explores the restrictions required to guarantee sound, complete and decidable type inference in the presence of functional dependencies.
 Multiple parameter classes in the Hugs 98 User Manual
 Problems with functional dependencies (email) by SPJ + paper. See also.
Tickets
Pros
 In GHC and Hugs for a long time.
 Used in important libraries, notably monad transformers.
 MultiParamTypeClasses are of limited use without functional dependencies or something equivalent.
Cons
 There are (at least) three different versions of FDs, none of which is satisfactory:
 Mark Jones's original proposal. Problem: It excludes some uses of FDs (see below).
 GHC's implementation. Problem: It makes type checking undecidable (see below).
 Chameleon's implementation. Problem: Needs type inference based on constraint handling rules (not just HM). Doesn't support separate compilation atm.
 Including the dependent type parameters makes types more cluttered, and prevents hiding of these types (see AssociatedTypes).
 AssociatedTypes seem to be more promising.
Variations
Original version
Suppose a class C has a functional dependency X > Y. The original paper imposed two restrictions on instances of the class C (sect. 6.1):
 For any instance
instance ... => C t
any variable occurring free in t_{Y} must also occur free in t_{X}.  If there is a second instance
instance ... => C s
then any substitution unifying t_{X} with s_{X} must also unify t_{Y} with s_{Y}.
Haskell 98 requires that the context of an instance declaration use only type variables that appear in the head. It was originally thought that this could be relaxed (original paper, sect. 6.3), to variables determined by those in the head, but this can lead to nontermination (CHR paper, ex. 16).
With FlexibleInstances and no OverlappingInstances, these restrictions yield a coherent and decidable system (CHR paper, corr. 1).
GHC and Hugs
GHC and Hugs implement a relaxed version of the first restriction above: they require only that any variable occurring free in t_{Y} be dependent (using dependencies of classes in the context) on variables that occur free in t_{X}. They thus accept instances like the following from the monad transformer library, which is invalid according to the original rules:
class (Monoid w, Monad m) => MonadWriter w m  m > w instance MonadWriter w m => MonadWriter w (ReaderT r m)
Unfortunately, this relaxation breaks the guarantees of termination and coherence:
 The following instances (CHR paper, ex. 6) seem reasonable:
class Mul a b c  a b > c where (.*.) :: a > b > c instance Mul Int Int Int where (.*.) = (*) instance Mul Int Float Float where x .*. y = fromIntegral x * y instance Mul a b c => Mul a [b] [c] where x .*. v = map (x.*.) v
and yet instance inference fails to terminate for the following (erroneous) definition:f = \ b x y > if b then x .*. [y] else y
 The following instances (adapted from CHR paper, ex. 18) are sensitive to the order in which rules are applied:
class B a b  a > b class C a b  a > b class D a b c  a > b where f :: a > b > c > Bool instance B a b => D [a] [b] Bool instance C a b => D [a] [b] Char
Given the constraint D [a] [b] Bool, D [a] [c] Char, if we apply the dependency first, and then reduce using the instances, we obtain b = c, B a b, C a b.
 if we first reduce using the instances, we obtain B a b, C a c.
New version
The following complex relaxation of the first rule is safe (CHR paper, sect. 6), and allows the instances in the monad transformer library:
 For any instance
instance ... => C t
either any variable occurring free in t_{Y} must also occur free in t_{X}, or
 the functional dependency is full (involves all the arguments of the class), and the arguments t_{Y} are type variables determined by the free variables of t_{X}.
Note that functional dependencies corresponding to associated type synonyms are always full.
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possible use case for FunctionalDependencies
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