If you've ever worked with dynamically typed languages, you may have had a taste of what type inferencing is. During runtime or compile time, your program's variables and expression types are deduced to a starting set of a given rules that describe some basic type associations. Generally, this field comes along with a wide range of academic theory and its inherent mechanics can reach a high level of complexity when advanced language constructions, such as lambda expressions or polymorphic structures, come into play.

As far as our recipe is concerned, we are going to implement a subset of an algorithm known as Hindley-Milner (or Damas-Milner) to keep things simple. In Hindley-Milner, we maintain an environment of known facts about types and, at every step, some rules are used to infer new information out of that environment up until the type of our root expression is known.

Actually, we will concern ourselves with two out of the six rules that make up the Hindley...