Functional Dependencies

Observe the type of (+):

GHCi session (+)

ghci> :t (+)
(+) :: forall a. Num a => a -> a

This is quite different in Python:

Python interactive session session (+)

>>> type(1 + 1)
class <'int'>
>>> type(1 + 1.0)
class <'float'>
>>> type(1.0 + 1)
class <'float'>
>>> type(1.0 + 1.0)
class <'float'>

The + operator in Python behaves heterogenously—when given two ints we get an int; when given at least one float we get a float. How would we encode this in Haskell?

Simple! Create a multi-parameter typeclass that describes the argument types and the result type!

HAdd.hs

class (Num a, Num b, Num c) => HAdd a b c where
  (+#) :: a -> b -> c

Then we can write instances for the possible permutations of the desired types:

HAdd.hs

instance Num a => HAdd a a a where
  (+#) :: a -> a -> a
  (+#) = (+)

instance HAdd Int Double Double where
  (+#) :: Int -> Double -> Double
  x +# y = fromIntegral x + y

instance HAdd Double Int Double where
  (+#) :: Double -> Int -> Double
  x +# y = x + fromIntegral y

However, trying to use (+#) is very cumbersome:

GHCi session (+#)

ghci> x :: Int = 1
ghci> y :: Double = 2.0
ghci> x +# y
<interactive>:3:1: error:
    - No instance for (HAdd Int Double ()) arising from a use of 'it'
    - In the first argument of 'print', namely 'it'
      In a stmt of an interactive GHCi command: print it
ghci> x +# y :: Double
3.0

This occurs because without specifying the return type c, Haskell has no idea what it is since it is ambiguous! As per the definition, no one is stopping us from defining another instance HAdd Int Double String! On the other hand, we know that adding an Int and a Double must result in a Double and nothing else; in other words, the types of the arguments to (+#) uniquely characterizes the resulting type.

The way we introduce this dependency between these type variables by introducing functional dependencies on typeclass declarations, which, adding them to our declaration of HAdd, looks something like the following:

HAdd.hs

{-# LANGUAGE FunctionalDependencies #-}
class (Num a, Num b, Num c) => HAdd a b c where
class (Num a, Num b, Num c) => HAdd a b c | a b -> c where
  (+#) :: a -> b -> c

The way to read the clause a b -> c is “a and b uniquely characterizes/determines c”, or in other words, c is a function of a and b, i.e. it is not possible that given a fixed a and b that we have two different inhabitants of c. This (1) prevents the programmer from introducing different values of c for the same a and b (which we haven’t) and (2) allows the compiler to infer the right instance just with a and b alone.

GHCi session (+#)

ghci> x :: Int = 1
ghci> y :: Double = 2.0
ghci> x +# y
3.0
ghci> :{
ghci| instance HAdd Int Double String where
ghci|   x +# y = show x
ghci| :}
<interactive>:8:10: error:
    Functional dependencies conflict between instance declarations:
      instance [safe] HAdd Int Double Double
        -- Defined at <interactive>:17:10
      instance HAdd Int Double String -- Defined at <interactive>:21:10