expression-problem.hs

{-
The expression problem: Adding new data types and new functionality to
existing data types is at odds.

OOP:

* adding new data types is easy
    - just write new code, and adhere to the API

* But adding new functionality to existing data types is hard
    - e.g. having to add `prettyPrint` to every class in the system
    - requires editing all existing classes

FP:
* adding new data types to the system is hard
    - all functions that act on those data types must change to accommodate the new type
* but adding new functionality to existing types is easy
    - just add a new function and pattern-match on existing data types

-}

-- Typeclasses solve the expression problem (ish):
-- Without changing existing code, we can make functions act on new datatypes
-- that conform to a typeclass.

-- Let's take this function:

-- myShow :: (Show a) => a -> String
-- myShow thing = "It's me: " ++ thing

-- Say we have this datatype:

data Fish = Carp | Salmon

-- And we want a function, `wow`, that takes these data types and says e.g.
-- "Wow, it's Carp", "Wow, it's Salmon", etc.
--
-- So we write this:

wow Carp = "Wow, it's Carp"
wow Salmon = "Wow, it's Salmon"

-- Now let's add another data type:
data Metal = Steel | Gold

-- Since we have a new type, we need to change `wow` -- which is *code we
-- already wrote* -- in order to make `wow` work with our
-- new data type:

wow Gold = "Wow, it's Gold"
wow Steel = "Wow, it's Steel"

-- On the flip side, Haskell makes adding a new function that acts on all
-- existing data types easy. We don't have to change any existing code, this is
-- 100% brand new:

notImpressed Carp = "Ugh, it's Carp"
notImpressed Salmon = "Ugh, it's Salmon"
notImpressed Gold = "Ugh, it's Gold"
notImpressed Steel = "Ugh, it's Steel"

-- But what about typeclasses? Type classes let us add new functionality
-- to existing types without changing existing code.
-- For example, let's assume we have just `Fish`, like before.

class Wow a where
    wow :: a -> String

data Fish = Carp | Salmon

instance Wow Fish where
    wow Carp = "Wow, it's Carp"
    wow Salmon = "Wow, it's Salmon"

-- Now we add Metal:

data Metal = Steel | Gold

-- We can add `wow` for Metal *without changing existing code*

instance Wow Metal where
    wow Steel = "Wow, it's Steel"
    wow Gold = "Wow, it's Gold"

-- Neat!