You may wish to study Alexis’s struct updater package:
> On May 26, 2018, at 11:46 AM, David Storrs <david.sto...@gmail.com> wrote:
> Hi folks,
> I am becoming more converted to the way of structs vs hashes as time goes on
> and so I sat down to write a macro that would generate functional setters as
> a convenience. Code is at bottom of email; I'd appreciate suggestions on how
> to improve it, in particular better ways of handling macros that have
> multiple optional parts, e.g.:
> (struct foo (bar baz jaz))
> (make-functional-setter foo bar)
> (make-functional-setter foo baz string?)
> (make-functional-setter foo jaz path-string? path-string->string)
> I've seen this handled in two ways:
> 1) define-syntax-class with multiple (pattern ...) clauses, which renders the
> parsing cleaner and easier to understand. Handling missing parts is still an
> issue, since using an attribute that wasn't matched is a syntax error, so my
> A) Is there a way to test if a syntax class has a particular attribute before
> trying to use it?
> B) Alternatively, is there a way to create a null syntax object that expands
> to nothing? Not (void), not "", literally nothing. Then I could have each
> pattern bind all the attributes via #:with and just have some of them be
> 2) Alternatively, macros with optional parts can be handled by a series of
> test cases in a syntax-parse, one for each option. That ends up with a lot
> of copy/pasted code between the segments. I'm getting around this via
> re-parsing but that feels clumsy. What is the better way?
> 3) There's something I'm not understanding about providing this thing. I
> have it in a file called make_functional_setter.rkt:
> #lang racket
> (require (for-syntax racket/syntax syntax/parse))
> (provide make-functional-setter)
> ; code and comments for make-functional-setter, see bottom of email
> When I (require "../make_functional_setter.rkt") on the CLI racket shell,
> everything works great. As soon as I try to require it into a file, it fails:
> Contents of file test.rkt:
> #lang racket
> (struct bar (a b))
> (require "../make_functional_setter.rkt")
> (make-functional-setter bar a)
> (make-functional-setter bar b path-string? ~a)
> When I do "racket test.rkt" on the command line, I get the following
> bar?: unbound identifier in module
> #(2082 module test 0) #(3901 module) #(3902 module struct 0) #(25633 macro)
> #(25639 use-site) #(25818 local) #(25820 intdef)
> other binding...:
> #(2081 module) #(2082 module test 0)
> in: bar?
> I've been through this in the DrRacket macro stepper but that hasn't helped
> me any. (It rarely does.)
> I'm assuming it's a phase issue, so I've played around with for-syntax,
> for-template, and for-meta but apparently I do not understand phases well
> What could be causing this?
> I've been going through all the macro- and syntax-related docs that I can
> find, but there's a lot of them and they take a few re-reads to sink in, so
> there's probably something exactly for this that I haven't seen.
> Code is below and at
> <https://gist.github.com/dstorrs/a53ec8736551eb5745d1d5fd265b5062> in case
> Gmail mangled the formatting.
> ;; make-functional-setter: macro for generating non-mutating field
> ;; setter functions for a struct
> ;; Define a struct: (struct book (title current-page filepath) #:transparent)
> ;; Generate 'set-book-title', 'set-book-current-page', and
> ;; All of these take two to four arguments: the 'book' struct to update, the
> ;; new value, an optional contract, and an optional wrapper function.
> ;; (make-functional-setter book title)
> ;; (make-functional-setter book current-page exact-positive-integer?)
> ;; (make-functional-setter book filepath path-string?
> ;; Details:
> ;; set-book-title accepts any value, regardless of sensibility
> ;; set-book-current-page accepts only exact-positive-integer?s, else
> contract violation
> ;; set-book-filepath accepts only path-string?s, converts to string
> before storing
> ;; Examples:
> ;; (define b (book "Foundation" 297 (build-path "/foo/bar")))
> ;; b ; (book "Foundation"
> 297 "/foo/bar")
> ;; (set-book-title b (hash)) ; (book (hash) 297
> ;; (set-book-current-page b 99) ; (book "Foundation"
> 99 "/foo/bar")
> ;; (set-book-current-page b 'x) ; ERROR! Contract
> ;; (set-book-filepath b (build-path "/foo")) ; (book "Foundation"
> 297 "/foo")
> (define-syntax (make-functional-setter stx)
> (syntax-parse stx
> ; First, grab the name of the struct and the field we're making
> ; this for. We'll build some stuff here then re-parse instead of
> ; copy/pasting for every pattern match
> [(_ type-name field-name ignored ...)
> (with-syntax* ([func-name (format-id #'type-name "set-~a-~a"
> #'type-name #'field-name)]
> [func-header #'(func-name the-struct val)]
> [definer #'define]
> [type-pred (format-id #'type-pred "~a?" #'type-name)]
> [func-body #'(struct-copy type-name the-struct
> [field-name val])]
> (syntax-parse stx
> [(_ _ _) #'(definer func-header func-body)]
> [(_ _ _ field-contract:expr ignored ...)
> (with-syntax ([definer #'define/contract]
> [func-contract #'(-> type-pred field-contract
> (syntax-parse stx
> [(_ _ _ _) #'(definer func-header func-contract func-body)]
> [(_ _ _ _ wrapper:expr)
> #'(definer func-header
> (struct-copy type-name the-struct [field-name (wrapper
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