Re: What should the constructor for a record look like?

[Matt Wette <matt.wette@gmail.com>]

On 08/26/2018 06:04 AM, HiPhish wrote:
> Hello, it's me again, the guy who wants to implement MessagePack [1] in Guile.
> The specification defines a type of "extension" [2], a pair of an 8-bit
> integer and a byte array for data. Implementing this type as a record is
> obvious, but what should be the name of the constructor?
>
>      (define-record-type ext
>        (make-ext type data)
>        ext?
>        (type ext-type)
>        (data ext-data))
>
> Either `make-ext` or just `ext` seem appropriate. I have seen both types of
> constructors, what do you guys say? And while I'm on the topic, what about
> types? An extension is only valid if the first field (type) is an integer from
> -128 to 127 and the data a vector of bytes (integer in the range from 0 to
> 255). How do I enforce this invariant without static typing?
>
> [1] https://msgpack.org/
> [2] https://github.com/msgpack/msgpack/blob/master/spec.md#ext-format-family
>
I like make-ext vs ext.

I assume you are using bytevectors for the data.  You could "enforce" that type in
the record by using a separate definition of `ext?'.

(define-record-type ext
   (make-ext type data)
   ext-record?
   (type ext-type)
   (data ext-data))

(define (ext? obj)
   (and (ext-record? obj)
        (integer? (ext-type obj))
        (bytevector? (ext-data obj)))