What is a type error?

[Chris Smith]

Darren New <dnew at san.rr.com> wrote:
> I'm not sure what linear or uniqueness typing is. It's typestate, and if 
> I remember correctly the papers I read 10 years ago, the folks at 
> TJWatson that invented Hermes also invented the concept of typestate. 
> They at least claim to have coined the term.

Coining the term is one thing, but I feel pretty confident that the idea 
was not invented in 1986 with the Hermes language, but rather far 
earlier.  Perhaps they may have invented the concept of considering it 
any different from other applications of types, though.  I still have 
trouble delineating how to consider "typestate" different from ordinary 
types in formal terms, unless I make the formal model complex enough to 
include some kind of programmer-defined identifiers such as variables.  
The distinction is not at all relevant to common type system models 
based on the lambda calculus.

While acknowledging, on the one hand, that the word "typestate" is used 
to describe this, I also object that types have *always* been assigned 
to expressions in differing type environments.  Otherwise, it would be 
impossible to assign types to lambda abstractions in the simply typed 
lambda calculus, the simplest of all generally studied type systems.  
What is being named here is the overcoming of a limitation that 
programming language designers imposed upon themselves, whether from not 
understanding the theoretical research or not believing it important, I 
don't know.

-- 
Chris Smith - Lead Software Developer / Technical Trainer
MindIQ Corporation