TS9001: No Instance Found

Category: Typeclass Resolution
Severity: Error

Message

No instance of '{typeclass}' found for type '{type}'

Explanation

The typesugar compiler couldn't find a typeclass instance to use at a call site. This happens when you:

1. Call summon<TC<T>>() for a typeclass TC and type T that doesn't have an instance
2. Use an operator like === or method like .show() on a type without the required typeclass instance
3. Have nested types where an inner field type lacks the required instance

Common Causes

1. Missing @derive annotation

// Error: No instance of 'Eq' found for type 'Point'
interface Point {
  x: number;
  y: number;
}

const p1: Point = { x: 1, y: 2 };
const p2: Point = { x: 1, y: 2 };
console.log(p1 === p2); // TS9001

Fix: Add @derive(Eq) to the type:

@derive(Eq)
interface Point {
  x: number;
  y: number;
}

2. Field type lacks instance

@derive(Eq)
interface Container {
  data: CustomType; // CustomType doesn't have Eq
}

Fix: Either derive Eq for CustomType, or provide a custom instance:

@derive(Eq)
class CustomType { /* ... */ }

// Or define a custom @instance (attaches to CustomType companion)
@instance
const eqCustomType: Eq<CustomType> = {
  equals: (a, b) => /* custom logic */
};
// Access via: CustomType.Eq

3. Generic type without constraint

function compare<T>(a: T, b: T): boolean {
  return summon<Eq<T>>().equals(a, b); // TS9001 if T has no Eq constraint
}

Fix: Add a typeclass constraint with = implicit():

// Option 1: Explicit parameter (caller must pass it)
function compare<T>(a: T, b: T, eq: Eq<T>): boolean {
  return eq.equals(a, b);
}

// Option 2: Use = implicit() to auto-fill (caller can still override)
function compare<T>(a: T, b: T, eq: Eq<T> = implicit()): boolean {
  return eq.equals(a, b);
}

Suppression

If you intentionally want to skip typeclass resolution for specific code, you can use the explicit form:

// Instead of relying on implicit resolution
p1 === p2;

// Use explicit equality check if you know it's safe
JSON.stringify(p1) === JSON.stringify(p2);

See Also

• Typeclass Guide
• Derive Guide
• TS9102 - Missing field instance