Add a way to create types in LBM code

[svenssonjoel]

There is no way (other than custom types) to add a new type to LBM.

Taking inspiration from ELISP defstruct and making use of LBM lisp-arrays we can come up with a way to create struct like types from LBM code.

(defstruct mystruct '(a b c)) could create the "shape of a struct" with three fields names a, b, c. When the defstruct is executed code will be generated:

1. mkmystruct such that (mkmystruct 1 2 3) => [| mystruct 1 2 3 |] where [| is a real lisp array. (also a zero arg version that initializes all members to nil)
2. mystruct? such that (mystruct? [| mystruct 1 2 3 |] => True (checks equality with index 0.
3. mystruct-a/b/c for projecting and setting depending on if you give them an argument of not?

This is a high-level early sketch of what is possible all input and insights are very welcome.

Potential downsides:

1. Each defstruct generates a bunch of symbols on the fly!
2. Each defstruct needs to generate a lookup table of symbol to index (unless we can think of something clever) for the accessor/setter operations.

[vedderb]

Do you have some example of something that becomes easier with this approach that can't be done easily today? As far as I know I haven't come across any situation where I miss something like this.

Also, would it be possible to make this in lisp alone without involving C?

[svenssonjoel]

This would fill the hole left after removing Custom Types (partially). It is not so much about making new things possible as it is to allow one to structure ones data.

Depending on how far one takes this, I think it could be a mostly lisp-code implementation.