### Question

How does one define a record?

The Dafny datatype feature lets you define simple or complex records, including recursive ones.

A simple enumeration record might be defined as

datatype ABCD = A | B | C | D


Variables of type ABCD can take one of the four given values.

A record might also be a single alternative but with data values:

datatype Date = Date(year: int, month: int, day: int)


where a record instance can be created like var d := Date(2022, 8, 23) and componenents retrieved like d.year.

There can be multiple record alternatives each holding different data:

datatype ABCD = A(i: int) | B(s: string) | C(r: real) | D
const a: ABCD := A(7)
const b: ABCD := B("asd")
const c: ABCD := C(0.0)
const d: ABCD := D


You can determine which alternative a record value is with the built-in test functions: with the definitions above, a.A? is true and b.C? is false. And you can extract the record alternative’s data: in the above a.i is well-defined if a.A? is true, in which case it has the value 7.

There is more description of datatypes here.