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Refine existing type in Predicate.isTagged #6497

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Refine existing type in Predicated.isTagged
thewilkybarkid Jul 20, 2026
7825337
Correct module name
thewilkybarkid Jul 20, 2026
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5 changes: 5 additions & 0 deletions .changeset/funny-drinks-scream.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"effect": patch
---

Refine existing type in Predicate.isTagged
4 changes: 2 additions & 2 deletions packages/effect/src/Predicate.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1164,8 +1164,8 @@ export const hasProperty: {
* @since 2.0.0
*/
export const isTagged: {
<K extends string>(tag: K): (self: unknown) => self is { _tag: K }
<K extends string>(self: unknown, tag: K): self is { _tag: K }
<K extends string>(tag: K): <T>(self: T) => self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
<K extends string, T>(self: T, tag: K): self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
Comment on lines +1167 to +1168

@coderabbitai coderabbitai Bot Jul 20, 2026

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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

🧩 Analysis chain

🏁 Script executed:

#!/usr/bin/env bash
set -euo pipefail

pnpm test-types packages/effect/typetest/Predicate.tst.ts
rg -n 'ReadonlyArray<object>|Record<string, unknown>|isTagged' packages/effect/typetest packages/effect/src/Predicate.ts

Repository: Effect-TS/effect

Length of output: 535

Preserve narrowing for broad object inputs.

T extends {} routes object and Record<string, unknown> through Extract, which collapses to never for values that can still pass the runtime _tag check. Use T extends { _tag: K } ? T : T & { _tag: K } in both overloads so tagged unions stay precise and broad inputs still narrow correctly.

Suggested refinement 
-  <K extends string>(tag: K): <T>(self: T) => self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
-  <K extends string, T>(self: T, tag: K): self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
+  <K extends string>(tag: K): <T>(self: T) => self is T extends { _tag: K } ? T : T & { _tag: K }
+  <K extends string, T>(self: T, tag: K): self is T extends { _tag: K } ? T : T & { _tag: K }
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
<K extends string>(tag: K): <T>(self: T) => self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
<K extends string, T>(self: T, tag: K): self is T extends {} ? Extract<T, { _tag: K }> : T & { _tag: K }
<K extends string>(tag: K): <T>(self: T) => self is T extends { _tag: K } ? T : T & { _tag: K }
<K extends string, T>(self: T, tag: K): self is T extends { _tag: K } ? T : T & { _tag: K }
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@packages/effect/src/Predicate.ts` around lines 1167 - 1168, Update both
overloads of the tag predicate to replace the T extends {} conditional with T
extends { _tag: K } ? T : T & { _tag: K }. Preserve precise narrowing for
already-tagged unions while allowing broad object inputs to narrow to the
required _tag shape.

Source: MCP tools

} = dual(
2,
<K extends string>(self: unknown, tag: K): self is { _tag: K } => hasProperty(self, "_tag") && self["_tag"] === tag
Expand Down
8 changes: 7 additions & 1 deletion packages/effect/typetest/Predicate.tst.ts
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Expand Up @@ -7,6 +7,8 @@ declare const unknowns: ReadonlyArray<unknown>
declare const numberOrNull: ReadonlyArray<number | null>
declare const numberOrUndefined: ReadonlyArray<number | undefined>
declare const numberOrNullOrUndefined: ReadonlyArray<number | null | undefined>
declare const tags: ReadonlyArray<{ _tag: "a"; a: string } | { _tag: "b"; b: string }>
declare const tagsOrNumbers: ReadonlyArray<{ _tag: "a"; a: string } | { _tag: "b"; b: string } | number>

describe("Predicate", () => {
it("isString", () => {
Expand Down Expand Up @@ -56,7 +58,11 @@ describe("Predicate", () => {
})

it("isTagged", () => {
expect(anys.filter(Predicate.isTagged("a"))).type.toBe<Array<{ _tag: "a" }>>()
expect(anys.filter(Predicate.isTagged("a"))).type.toBe<Array<any>>()
expect(unknowns.filter(Predicate.isTagged("a"))).type.toBe<Array<{ _tag: "a" }>>()
expect(tags.filter(Predicate.isTagged("a"))).type.toBe<Array<{ _tag: "a"; a: string }>>()
expect(tagsOrNumbers.filter(Predicate.isTagged("a"))).type.toBe<Array<{ _tag: "a"; a: string }>>()
expect(numberOrNull.filter(Predicate.isTagged("a"))).type.toBe<Array<never>>()
})

it("isNullish", () => {
Expand Down