Schema.transform does not preserve branding through encode/decode

[SimonVadier]

What version of Effect is running?

3.19.5

What steps can reproduce the bug?

When using Schema.transform with branded types, the brand is lost during the encode operation. The From/To branding is not preserved when encoding back from the transformed type.

import * as Schema from "effect/Schema";

const BrandedBool = Schema.Boolean.pipe(Schema.brand("bool"));
const BrandedString = Schema.String.pipe(Schema.brand("string"));

const BoolFromString = Schema.transform(BrandedString, BrandedBool, {
  strict: false,
  decode: (i) => i === "on",
  encode: (bool) => (bool ? "on" : "off"),
});

const bool = BrandedBool.make(false); // boolean & Brand<"bool">
const stringFromBool = Schema.encodeSync(BoolFromString)(bool); // Expected: string & Brand<"string">, Actual: string
const boolFromString = Schema.decodeSync(BoolFromString)(stringFromBool); // boolean & Brand<"bool">

What is the expected behavior?

Schema.encodeSync(BoolFromString)(bool) should return string & Brand<"string"> because BrandedString is used as the From type in the transformation.

What do you see instead?

Schema.encodeSync(BoolFromString)(bool) returns string without the brand. The Brand<"string"> is stripped during encoding.

Additional information

No response

[gcanti]

that's intended, if you want the branded type on the encoded side you can use Schema.typeSchema in the transformation

const BoolFromString = Schema.transform(Schema.typeSchema(BrandedString), BrandedBool, {
  strict: false,
  decode: (i) => i === "on",
  encode: (bool) => (bool ? "on" : "off"),
});

[SimonVadier]

I hadn’t realized that Schema.typeSchema(BrandedString) has the type Schema.Schema<string & Brand<"string">, string, never>, that clears things up.

Out of curiosity, why not keep the branding on the encoded side by default?

Thanks for the help!

[gcanti]

IMO it's a sensible default. Also, from a practical point of view, the current behavior supports both use cases:

• using the unbranded type on the encoding side (the default)
• your use case through Schema.typeSchema (less common IME)