Does Haskell have return type overloading?

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Well, one way to look at it is that Haskell translates the return type polymorphism that you’re thinking of into parametric polymorphism, using something called the dictionary-passing translation for type classes. (Though this is not the only way to implement type classes or reason about them; it’s just the most popular.)

Basically, fromInteger has this type in Haskell:

fromInteger :: Num a => Integer -> a

That might be translated internally into something like this:

fromInteger# :: NumDictionary# a -> Integer -> a
fromInteger# NumDictionary# { fromInteger = method } x = method x

data NumDictionary# a = NumDictionary# { ...
                                       , fromInteger :: Integer -> a
                                       , ... }

So for each concrete type T with a Num instance, there’s a NumDictionary# T value that contains a function fromInteger :: Integer -> T, and all code that uses the Num type class is translated into code that takes a dictionary as the argument.

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