It's harder for "read" because there is a tangle of auxiliary functions. But for Int, for example, it's essentially the same function you would call "atoi" in C: A function that converts a string like "-123" into the integer -123, or signals an error if the input string is not valid.
But I think there may be a very important misunderstanding here. Neither you nor Haskell define THE read function. If you define a new type, you can define a new read function for this type. So if you define some type MyOwnT, you will define a read function of the concrete type "read :: String -> MyOwnT".
Nobody ever defines a "master" generic function "read :: String -> a" that can return any type! There are many concrete read functions, and the compiler chooses, based on type inference and type hints and sometimes type information at runtime, which concrete one to use at that particular call site.
Again: There is no single magic function that knows how to fabricate an instance of any type from a string. There is only a family of non-magic functions that each know how to fabricate an instance of a specific type.
Here is the implementation for mempty for the "Sum" type family mentioned in the article:
https://github.com/ghc/ghc/blob/d987f71aa3200ce0c94bc57c43b4...And here it is for lists:
https://github.com/ghc/ghc/blob/d987f71aa3200ce0c94bc57c43b4...It's harder for "read" because there is a tangle of auxiliary functions. But for Int, for example, it's essentially the same function you would call "atoi" in C: A function that converts a string like "-123" into the integer -123, or signals an error if the input string is not valid.
But I think there may be a very important misunderstanding here. Neither you nor Haskell define THE read function. If you define a new type, you can define a new read function for this type. So if you define some type MyOwnT, you will define a read function of the concrete type "read :: String -> MyOwnT".
Nobody ever defines a "master" generic function "read :: String -> a" that can return any type! There are many concrete read functions, and the compiler chooses, based on type inference and type hints and sometimes type information at runtime, which concrete one to use at that particular call site.
Again: There is no single magic function that knows how to fabricate an instance of any type from a string. There is only a family of non-magic functions that each know how to fabricate an instance of a specific type.