Functional Programming: A PragPub Anthology

The Language of Haskell’s Type System

I suggested that one important feature for a type system is how much it allows us to describe, so we’ll start looking at the language of Haskell types first. It’s perfectly accurate and appropriate to describe a type system as a language—it has words, grammar rules, and meanings. However, languages differ in what they can be used to express (or articulate), by virtue of the forms of sentences available.

Propositional logic can only talk about atomic propositions, like “Rubies are red,” and combinations of these via the logical operators—for example, “Rubies are red and not(Rails is written in C).” Predicate logic allows some more flexibility by allowing the ability to quantify over entities, hence enabling ...