Product: fix tuple and struct covariant params (account for empty dat… (

#11)
Effect-TS · Jul 29, 2023 · bfb24ad · bfb24ad
1 parent d4ecf44
commit bfb24ad
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Showing 4 changed files with 24 additions and 8 deletions.
diff --git a/.changeset/weak-scissors-breathe.md b/.changeset/weak-scissors-breathe.md
@@ -0,0 +1,5 @@
+---
+"@effect/typeclass": patch
+---
+
+Product: fix tuple and struct covariant params (account for empty data structures)
diff --git a/docs/modules/Product.ts.md b/docs/modules/Product.ts.md
@@ -48,8 +48,8 @@ export declare const struct: <F extends TypeLambda>(
 ) => Kind<
   F,
   [R[keyof R]] extends [Kind<F, infer R, any, any, any>] ? R : never,
-  [R[keyof R]] extends [Kind<F, any, infer O, any, any>] ? O : never,
-  [R[keyof R]] extends [Kind<F, any, any, infer E, any>] ? E : never,
+  R[keyof R] extends never ? never : [R[keyof R]] extends [Kind<F, any, infer O, any, any>] ? O : never,
+  R[keyof R] extends never ? never : [R[keyof R]] extends [Kind<F, any, any, infer E, any>] ? E : never,
   { [K in keyof R]: [R[K]] extends [Kind<F, any, any, any, infer A>] ? A : never }
 >
 ```
@@ -68,8 +68,8 @@ export declare const tuple: <F extends TypeLambda>(
 ) => Kind<
   F,
   [T[number]] extends [Kind<F, infer R, any, any, any>] ? R : never,
-  [T[number]] extends [Kind<F, any, infer O, any, any>] ? O : never,
-  [T[number]] extends [Kind<F, any, any, infer E, any>] ? E : never,
+  T[number] extends never ? never : [T[number]] extends [Kind<F, any, infer O, any, any>] ? O : never,
+  T[number] extends never ? never : [T[number]] extends [Kind<F, any, any, infer E, any>] ? E : never,
   { [I in keyof T]: [T[I]] extends [Kind<F, any, any, any, infer A>] ? A : never }
 >
 ```

diff --git a/dtslint/Product.ts b/dtslint/Product.ts
@@ -21,5 +21,8 @@ _.tuple(Product)(fa, fb, fc)
 // $ExpectType RAW<{ a: string; } & { b: number; } & { c: boolean; }, "a" | "b" | "c", { fa: string; fb: number; fc: boolean; }>
 _.struct(Product)({ fa, fb, fc })
 
+// $ExpectType RAW<unknown, never, []>
 _.tuple(Product)() // should allow empty tuple
+
+// $ExpectType RAW<unknown, never, {}>
 _.struct(Product)({}) // should allow empty structs
diff --git a/src/Product.ts b/src/Product.ts
@@ -23,8 +23,12 @@ export const tuple = <F extends TypeLambda>(F: Product<F>) =>
   <T extends ReadonlyArray<Kind<F, any, any, any, any>>>(...elements: T): Kind<
     F,
     ([T[number]] extends [Kind<F, infer R, any, any, any>] ? R : never),
-    ([T[number]] extends [Kind<F, any, infer O, any, any>] ? O : never),
-    ([T[number]] extends [Kind<F, any, any, infer E, any>] ? E : never),
+    (T[number] extends never ? never
+      : [T[number]] extends [Kind<F, any, infer O, any, any>] ? O
+      : never),
+    (T[number] extends never ? never
+      : [T[number]] extends [Kind<F, any, any, infer E, any>] ? E
+      : never),
     { [I in keyof T]: [T[I]] extends [Kind<F, any, any, any, infer A>] ? A : never }
   > => F.productAll(elements) as any
 
@@ -35,8 +39,12 @@ export const struct = <F extends TypeLambda>(F: Product<F>) =>
   <R extends { readonly [x: string]: Kind<F, any, any, any, any> }>(fields: R): Kind<
     F,
     ([R[keyof R]] extends [Kind<F, infer R, any, any, any>] ? R : never),
-    ([R[keyof R]] extends [Kind<F, any, infer O, any, any>] ? O : never),
-    ([R[keyof R]] extends [Kind<F, any, any, infer E, any>] ? E : never),
+    (R[keyof R] extends never ? never
+      : [R[keyof R]] extends [Kind<F, any, infer O, any, any>] ? O
+      : never),
+    (R[keyof R] extends never ? never
+      : [R[keyof R]] extends [Kind<F, any, any, infer E, any>] ? E
+      : never),
     { [K in keyof R]: [R[K]] extends [Kind<F, any, any, any, infer A>] ? A : never }
   > => {
     const keys = Object.keys(fields)