fix(Match): handle null/undefined in Match.tag and Match.tagStartsWith (#6018)

Author: codewithkenzo

.changeset/fix-match-tag-nullable.md

@@ -0,0 +1,9 @@
+---
+"effect": patch
+---
+
+fix(Match): handle null/undefined in `Match.tag` and `Match.tagStartsWith`
+
+Added null checks to `discriminator` and `discriminatorStartsWith` predicates to prevent crashes when matching nullable union types.
+
+Fixes #6017

packages/effect/src/internal/matcher.ts

@@ -351,8 +351,8 @@ export const discriminator = <D extends string>(field: D) =>
   const f = pattern[pattern.length - 1]
   const values: Array<P> = pattern.slice(0, -1) as any
   const pred = values.length === 1
-    ? (_: any) => _[field] === values[0]
-    : (_: any) => values.includes(_[field])
+    ? (_: any) => _ != null && _[field] === values[0]
+    : (_: any) => _ != null && values.includes(_[field])
 
   return <I, F, A, Pr>(
     self: Matcher<I, F, R, A, Pr, Ret>
@@ -377,7 +377,7 @@ export const discriminatorStartsWith = <D extends string>(field: D) =>
   pattern: P,
   f: Fn
 ) => {
-  const pred = (_: any) => typeof _[field] === "string" && _[field].startsWith(pattern)
+  const pred = (_: any) => _ != null && typeof _[field] === "string" && _[field].startsWith(pattern)
 
   return <I, F, A, Pr>(
     self: Matcher<I, F, R, A, Pr, Ret>

packages/effect/test/Match.test.ts

@@ -207,6 +207,36 @@ describe("Match", () => {
     doesNotThrow(() => match(undefined))
   })
 
+  it("Match.tag with nullable union", () => {
+    const match = M.type<{ _tag: "A" } | undefined>().pipe(
+      M.tag("A", () => "matched A"),
+      M.when(undefined, () => "matched undefined"),
+      M.exhaustive
+    )
+    strictEqual(match({ _tag: "A" }), "matched A")
+    strictEqual(match(undefined), "matched undefined")
+  })
+
+  it("Match.tag with null union", () => {
+    const match = M.type<{ _tag: "A" } | null>().pipe(
+      M.tag("A", () => "matched A"),
+      M.when(null, () => "matched null"),
+      M.exhaustive
+    )
+    strictEqual(match({ _tag: "A" }), "matched A")
+    strictEqual(match(null), "matched null")
+  })
+
+  it("Match.tagStartsWith with nullable union", () => {
+    const match = M.type<{ _tag: "A.B" } | undefined>().pipe(
+      M.tagStartsWith("A", () => "matched A prefix"),
+      M.when(undefined, () => "matched undefined"),
+      M.exhaustive
+    )
+    strictEqual(match({ _tag: "A.B" }), "matched A prefix")
+    strictEqual(match(undefined), "matched undefined")
+  })
+
   it("discriminator multiple", () => {
     const result = pipe(
       M.value(Either.right(0)),