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
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