On Fri, Mar 9, 2018 at 9:35 PM, Kevin Forchione <lyss...@gmail.com> wrote:
> Is it possible to initialize a struct field based on values from
> previously defined fields? Something equivalent to let* where
>
> >(struct foo (A B C))
> >(foo 1 2) would produce (foo 1 2 3) for example?
>
>
> As far as I know, the only way to do this is to write your own function
that calls the struct constructor. You can do this in a module, and only
export your constructor, instead of the default struct constructor.
If you want the constructor and the match expander to use the same
identifier, then you need to do a bit more work. For example:
```
#lang racket/base
(require racket/match
(for-syntax racket/base
syntax/transformer))
(struct foo* (A B C))
(define (foo a b)
(foo* a b (+ a b)))
(define-match-expander $foo
(syntax-rules ()
[(foo a b c) (foo* a b c)])
(make-variable-like-transformer #'foo))
(provide (rename-out [$foo foo]
[foo*-A foo-A]
[foo*-B foo-B]
[foo*-C foo-C])))
```
Of course, even here, an instance will print as #<foo*>. You can implement
gen:custom-write to fix that.
There might be a better way to do this, but this is what I've used in the
past.
- Jon
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