I'm trying to use the json
package in Typed Racket, but I'm having some trouble handling how to type the jsexpr?
predicate. My first attempt was simply using #:opaque
.
(require/typed json
[#:opaque JSExpr jsexpr?])
The trouble is that a jsexpr
is not a struct, jsexpr?
is simply a predicate that tests whether or not a given value fits a certain structure. In truth, a JSExpr
type should look something like this.
(define-type JSExpr (U
'null Boolean String Integer Inexact-Real
(Listof JSExpr) (HashTable Symbol JSExpr)))
So then, I would just use that JSExpr
type, but there's still a problem. Now I have a (U JSExpr EOF)
type, and I need to convert it to a JSExpr
type (I want to throw an exception if I get EOF). Therefore, I want to do something like this:
(cond
[(jsexpr? json-data) json-data]
[else (error "failed to parse JSON data")])
This should work with Racket's occurrence typing, but now I don't have jsexpr?
defined! Fortunately, there exists define-predicate
to generate that function for me. Unfortunately, it doesn't work with the JSExpr
type because a predicate needs a flat contract, and potentially-mutable data structures such as HashTable
require chaperone contracts.
Alright, well what about typing the actual jsexpr?
predicate to occurrence-type for JSExpr
?
(require/typed json
[jsexpr? (-> Any Boolean : JSExpr)])
Unfortunately, this doesn't work, either, because filters can't be used in require/typed
.
I understand that the real problem is most likely stemming from the fact that HashTable
is mutable, but that's not something I can change. I suppose I could write my own (-> Any Boolean : JSExpr)
function, but that would sort of defeat the purpose of using the library.
Is there any way at all to make this work? A simple ImmutableHashTable
type would likely suffice here, but that doesn't seem to exist.
From the mailing list:
The problem is that typed racket doesn’t know that a value of type String for instance will be of type JSExpr or not, so you would have to put
(assert x jsexpr?)
around everything that you want to treat as a jsexpr.
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