withDefault alters S.is returned value

[Vincenzo Chianese]

Long time no chat @gcanti!

I am using Effect-TS in Microsoft (and really every single company I have worked at and will work at) and I am having a weird behavior that I believe might be a bug.

It sounds like using withDefault alters the return value of S.is - and I do not know if I am doing something wrong or it is a bug.

What version of @effect/schema is running?

0.50.0

What steps can reproduce the bug?

const S = require("@effect/schema/Schema");
const struct = S.struct({
  skip: S.optional(S.array(S.string)).withDefault(() => []),
});

S.parseSync(struct)({});
// { skip: [] }

S.is(struct)({});
// false

const struct2 = S.struct({ skip: S.optional(S.array(S.string)) });

S.parseSync(struct2)({});
// {}
S.is(struct2)({});
//true

What is the expected behavior?

I would be expecting to see true for the first part as well.

[Giulio Canti]

It's working as intended: applying a default is a transformation carried out by, for example, parseSync. However, S.is doesn't perform any transformation, it simply verifies that the input conforms to a type (in this case, { readonly skip: readonly string[]; }).

[Vincenzo Chianese]

That I understand, and I am not asking to is to apply the transformation. What is unclear is - if skip is optional, why is not the empty object passing the test? Why is withOptional changing the behavior?

[Giulio Canti]

skip is optional in the From type but required in the To type:

import * as S from "@effect/schema/Schema"

/*
const struct: S.Schema<{
    readonly skip?: readonly string[]; // From type
}, {
    readonly skip: readonly string[]; // To type
}>
*/
const struct = S.struct({
  skip: S.optional(S.array(S.string)).withDefault(() => [])
})

and S.is verifies the To type as you can see from its signature

const is: <_, A>(schema: Schema.Schema<_, A>) => (a: unknown) => a is A

[Vincenzo Chianese]

Oooj ok - that was my misunderstanding then. Thanks a lot for the prompt reply!!