Parametric. In ML-family languages (SML, OCaml, Haskell) you can define a function that pattern-matches its signature. For example (taken from the Wikipedia ar

Parametric. In ML-family languages (SML, OCaml, Haskell) you can define a function that pattern-matches its signature. For example (taken from the Wikipedia article on SML) I could define an algebraic datatype "shape" as such: datatype shape = Circle of loc * real (* center and radius *) | Square of loc * real (* upper-left corner and side length; axis-aligned *) | Triangle of loc * loc * loc (* corners *) Then I could define an "area" function that takes this as a parameter and pattern-matches on its type as such, with a different implementation depending on the type: fun area (Circle (_, r)) = 3.14 * r * r | area (Square (_, s)) = s * s | area (Triangle (a, b, c)) = heron (a, b, c) (* call to Heron's formula function *) Which is also syntactic sugar for: fun area shape = case shape of Circle (_, r) => 3.14 * r * r | Square (_, s) => s * s | Triangle (a, b, c) => heron (a, b, c) If I were to implement the function like: fun area shape = case shape of Circle (_, r) => 3.14 * r * r | Square (_, s) => s * s and forget to include the Triangle case, I would get a compiler error: "match nonexhaustive". Furthermore if I were to include a redundant case or an unreachable case, the compiler would also throw an error informing me of that. Very useful in preventing runtime errors, and languages like Java or C/C++ simply do not provide this.