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arktype•2y ago

Runtime coercion

I'm building a system to load declarative runtime configuration from a couple different sources, all of which provide values as strings (env vars, AWS SSM Parameters). Currently I'm walking through the typeJson at runtime to build accessors for each field, such as joining field names with __ to build environment variable keys, and that is working great. I can cleanly detect when I reach a "leaf" type to load, and I either have a string or an object with a "domain"; my challenge is whether there is an elegant way of coercing the string I load into the type expected at this point (without making some huge imperative logic block). I'm hopeful there might be some clever thing under the hood that I can use in place of rolling my own. Also, are there types defined somewhere that more concretely describe the typeJson structure?

29 Replies

ssalbdivad•2y ago

Hey, cool to hear you've been delving into some of the type system in depth! The easiest thing would be to traverse the type nodes directly instead of the JSON. If you create a type like:

const t = type({
   foo: "number"
}).or({
   foo: "string"
})

// access internal nodes
console.log(t.raw)
console.log(t.raw.kind)
console.log(t.raw.references)

if(t.raw.hasKind("union")) {
    console.log(t.raw.discriminant)
}

const t = type({
   foo: "number"
}).or({
   foo: "string"
})

// access internal nodes
console.log(t.raw)
console.log(t.raw.kind)
console.log(t.raw.references)

if(t.raw.hasKind("union")) {
    console.log(t.raw.discriminant)
}

I'm not totally sure what you mean about coercing the string into an expected type, maybe an example would help?

SneakyTurtleOP•2y ago

Sure! (also, thanks for the quick reply!) So, I want to define a "config" type like

const MyConfig = type({
  server:{ base_url: "url" }
})

const MyConfig = type({
  server:{ base_url: "url" }
})

and then introspect that type to load the value from process.env.SERVER__BASE_URL. This works fine currently if the type (url in this case) is a string-based type, but if I want to load a number, I need a way to coerce into the base type (before any validators)

ssalbdivad•2y ago

Maybe something like this would be helpful:

        const user = type({
            name: "string",
            age: "number"
        })
        const parsedUser = type("string").pipe(s => JSON.parse(s), user)

        const user = type({
            name: "string",
            age: "number"
        })
        const parsedUser = type("string").pipe(s => JSON.parse(s), user)

It's not like Zod where there's a "before validation" phase or similar, types can be chained through transformations to other types There are also some builtin keywords that could be helpful here like parse.number

SneakyTurtleOP•2y ago

Oh that sounds interesting where can I get parse.number?

ssalbdivad•2y ago

It should be available by default

SneakyTurtleOP•2y ago

As a global?

ssalbdivad•2y ago

Like

type("parse.number")

type("parse.number")

I mean You can also import ark And use ark.parse.number

SneakyTurtleOP•2y ago

Oh I see still wrapping my head around using type like that

ssalbdivad•2y ago

The completions should help a lot

SneakyTurtleOP•2y ago

yeah, those are super nifty

ssalbdivad•2y ago

And very specific error messages if you get anything wrong

SneakyTurtleOP•2y ago

kudos on building such an intricate type machine

ssalbdivad•2y ago

The best thing about the syntax IMO is that once you call type once you don't have to keep wrapping things You can define an object as deep as you want within a single type call

SneakyTurtleOP•2y ago

how would I go about parsing a comma separated string into an array of things

ssalbdivad•2y ago

So it looks much more like pure TS syntax

const t = type("string").pipe(s => s.split(","))

const t = type("string").pipe(s => s.split(","))

Then whatever additional transforms/validation you want on each item

SneakyTurtleOP•2y ago

I'm guessing thats what I would use a morph for?

ssalbdivad•2y ago

Yeah Pipe accepts N morphs. Where another Type is basically just a morph that will stop chaining if there are errors if you need intermediate validation

SneakyTurtleOP•2y ago

🤯

ssalbdivad•2y ago

I hope once people get the hang of the first few concepts everything is very composable and intuitive from there. Working on new docs now 🙏

SneakyTurtleOP•2y ago

Should this work as an inline morph:

list_o_strings: ['string', '|>', (s:string)=>s.split(',').map(x=>x.trim())],

list_o_strings: ['string', '|>', (s:string)=>s.split(',').map(x=>x.trim())],

ssalbdivad•2y ago

Which version are you using?

SneakyTurtleOP•2y ago

2.0.0-dev.18 I believe

ssalbdivad•2y ago

Ahh okay that was the alpha morph operator for inlining, you want => in 2.0

SneakyTurtleOP•2y ago

ahh, that makes more sense too

ssalbdivad•2y ago

You shouldn't need to explicitly type string although there are limits to what TS will do with some nested tuples You can also always define the morph type on its own, then reference it

SneakyTurtleOP•2y ago

I think it was just lacking inferrence because of the wrong operator in the middle yep, works without the explicit string

ssalbdivad•2y ago

const splitAndTrim = type("string").pipe(s => s.split(",").map(s => s.trim()))
const obj = type({
    list_o_strings: splitAndTrim
})

const splitAndTrim = type("string").pipe(s => s.split(",").map(s => s.trim()))
const obj = type({
    list_o_strings: splitAndTrim
})


Or you can inline the type call, whatever is easiest (although I've found defining them as separate variables is nice for reusability, readability, and TS has an easier time with it)
Or tuple expressions, it's really just the nested

type` calls that TS doesn't like

SneakyTurtleOP•2y ago

One last question for you @ssalbdivad (though I see you're offline for now, so no rush) If I wanted to do listOfInts how should I go about adding validation? Just doing

type('string').pipe((s) => s.split(',').map((x) => parseInt(x.trim())));

type('string').pipe((s) => s.split(',').map((x) => parseInt(x.trim())));

allows for NaN to slip into the array I tried type('string').pipe((s) => s.split(',').map((x) => type('integer')(x.trim()))); but that felt clunky

ssalbdivad•2y ago

Maybe this?

type("string").pipe(s => s.split(",").map(x => x.trim()), type("parse.integer"))

type("string").pipe(s => s.split(",").map(x => x.trim()), type("parse.integer"))

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