Functional Programming: A PragPub Anthology

What Are Types For?

Types do sometimes get some bad press. Many people will say types are about nailing details down and trying to avoid errors. This sounds to me a bit dry and negative, too much like a straitjacket, too conservative.

Some type systems undoubtedly are too restrictive and just don’t give enough return for the work required to use them (Java is a prime example), but I’m talking about Haskell’s type system here, which are quite a different kettle of fish. In this context, I prefer to see and use types as both a design language and a safety net. That is, they are more about help and opportunity rather than constraint, and this is pretty much how the type systems have been designed into these modern languages.

Design-wise, I ...