2016-08-02 16:05:27 UTC
I've found myself running into a few situations where I want to define a
type as "an object of (existing type a) for which (boolean expression is
Suppose, for example, that I want to represent identifiers in a language
I'm trying to model. Naturally, I'd think of using the primitives
defined in string.thy, but as not all strings are valid identifiers, I'd
define a function such as
fun is_identifier :: "string => bool"
But thereafter I'd need to code it in lemmas as taking is_identifier as
an assumption. I'd tend to think it'd be neater to have identifier being
a type defined as "string which satisfies is_identifier", and was
wondering if there was a way to do this?