Thanks Wesley and Matthias! Very helpful advice.
March 31, 2020 2:40 AM, "Wesley Bitomski" <wesley.bitom...@gmail.com
(mailto:wesley.bitom...@gmail.com?to=%22Wesley%20Bitomski%22%20<wesley.bitom...@gmail.com>)>
wrote:
Hello Ed,
I generally get some mileage out of using custom predicates (those that are
made with define-predicate) and pattern matching when tearing into JSON
structures in Typed Racket. Occurrence typing seems to work with if, cond and
assert, along with match's predicate matcher (? form).
So, something like this I think could less tedious to produce, and does exactly
what your example does:
(: jsexpr->cd-users-data (JSExpr -> (Option (Listof String))))
(define (jsexpr->cd-users-data js)
(define-predicate names-list? (Listof String))
(match js
[(hash-table ['Type "DTHidden"]
['Data (hash-table ['Type "CDUsers"]
['Data (? names-list? users)])])
users]
[_ #false]))
I hope this is what you were looking for.
On Sunday, March 29, 2020 at 8:39:56 AM UTC-4, e...@disroot.org wrote:
Hi everyone,
Recently I've been experimenting with Typed Racket and trying to
gradually type my code base.
One of the functions that I need to write is to extract a list of strings from
a JSON object, if it has following form:
{
"Type": "DTHidden",
"Data": { "Type": "CDUsers",
"Data": ["datum1", "datum2"] }
}
The way I used to structure it in Racket is to have a function
`is-cdusers?` with the contract `jsexpr? -> boolean?`, which would check that
the JSON object has the right shape; and a separate function `get-cdusers-data`
with the contract `is-cdusers? -> listof string?`.
However, after playing a bit with Typed Racket I decided that it was
necessary, according to my understanding of occurrence typing, to have a single
function `get-cdusers-data` with the type `JSExpr -> (U False (Listof String))`.
In order to get it to work I ended up writing a long chain of conditionals:
(: get-cdusers-data (-> JSExpr (U False (Listof Any))))
(define (get-cdusers-data js)
(if (and (hash? js)
(equal? DTHidden (hash-ref js 'Type #f)))
(let ([js (hash-ref-def js 'Data [ann #hasheq() JSExpr])])
(if (and (hash? js)
(equal? CdUsers (hash-ref js 'Type #f)))
(let ([data (hash-ref js 'Data)])
(if (hash? data)
(let ([x (hash-ref js 'Data #f)])
(and (list? x) x))
#f))
#f))
#f))
Needless to say, this is a bit impractical and error-prone to write.
Does anyone know if there is a better approach to this?
From my experience with typed languages I would get that the most
principle approach is to have an algebraic data type that represents all the
underlying data structures, something like
type reply = ... | CDUsers of string list | ...
and then have a single function to converts a JSExpr into that data
type.
I was hoping to avoid that, because I do enjoy working with the JSExpr
type directly in Racket.
Does anyone have advice/experience with problems like this?
Best wishes,
-Ed--
You received this message because you are subscribed to the Google Groups
"Racket Users" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to racket-users+unsubscr...@googlegroups.com
(mailto:racket-users+unsubscr...@googlegroups.com).
To view this discussion on the web visit
https://groups.google.com/d/msgid/racket-users/f85a4a4d-0d0a-47e3-909b-2b378def368a%40googlegroups.com
(https://groups.google.com/d/msgid/racket-users/f85a4a4d-0d0a-47e3-909b-2b378def368a%40googlegroups.com?utm_medium=email&utm_source=footer).
--
You received this message because you are subscribed to the Google Groups
"Racket Users" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to racket-users+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/racket-users/e678bbf9d87ef3c72db8ed45f9459c60%40disroot.org.
