Question

Both concepts allow new data types to be created.
The only difference I can see is that in functional languages, one can perform pattern matching on algebraic data types. But there is no comparable succinct feature for OO languages. Is this an accurate statement ?

Answer 1

I can see three major differences between algebraic data types and OO-style classes, not counting (im)mutablility because that varies.

• Algebraic data types allows sums as well as products, whereas OO-style classes only allow products.
• OO-style classes allow you to bundle a complex data item with it’s accepted operations, whereas algebraic data types don’t.
• Algebraic data types don’t distinguish between the data passed to the constructor and the data stored in the resulting value, whereas OO-style classes do (or can).

One thing I deliberately left out of that list was subtyping. While the vast majority of OO languages allow you to subclass (non-final, non-sealed, currently accessible) classes, and the vast majority of generally ML-family functional languages do not, it is clearly possible to forbid inheritance completely in a hypothetical OO (or at least OO-like) language, and it is likewise possible to produce subtyping and supertyping in algebraic data types; for a limited example of the latter, see this page on O’Haskell, which has been succeeded by Timber