The Expression Problem

Read those references if you are not familiar yet (TODO: maybe add tl;dr; here)

• https://wiki.c2.com/?ExpressionProblem
• https://en.wikipedia.org/wiki/Expression_problem

I thought type classes solve the problem nicely. Here is the example in Haskell using type class

data Shape = Square Float | Circle Float
​
area :: Shape -> Float
area x = case x of
  Square side -> (side * side)
  Circle radius -> (3.14 * radius * radius)
​

“Solution” using Type Class

​
data Square1 = Square1 Float
data Circle1 = Circle1 Float
​
class Area a where
  area1 :: a -> Float
​
instance Area Square1 where
  area1 (Square1 side) = side * side
​
instance Area Circle1 where
  area1 (Circle1 r) = 3.14 * r * r

Thanks to Daniel, he pointed me that type class still has limitations. For instance

TODO: demo the limitation

I guess it is because Type Class doesn’t really solve the problem directly but work around a bit.

The original problem/ask is to extend a data type but with type class, that original (single) data type has been broken into multiple data types hence the limitation.

I gave another thought when comparing the OO approach (easy data extension but not operation extension), operations are part of the Class (data type) hence it is never a question at table to talk about operations for data type interaction.

• Read once but didn’t follow well. Maybe worth re-read.