refactor: also move ewma+half-open check into option

breaker.go

@@ -1,6 +1,7 @@
 package hoglet
 
 import (
+	"fmt"
 	"math"
 	"sync/atomic"
 	"time"
@@ -123,6 +124,14 @@ func (e *EWMABreaker) observe(halfOpen, failure bool) stateChange {
 	}
 }
 
+// apply implements Option.
+func (e *EWMABreaker) apply(o *options) error {
+	if o.halfOpenDelay == 0 {
+		return fmt.Errorf("EWMABreaker requires a half-open delay")
+	}
+	return nil
+}
+
 // SlidingWindowBreaker is a [Breaker] that uses a sliding window to determine the error rate.
 type SlidingWindowBreaker struct {
 	windowSize time.Duration
@@ -208,8 +217,6 @@ func (s *SlidingWindowBreaker) observe(halfOpen, failure bool) stateChange {
 	}
 }
 
-var _ Option = (*SlidingWindowBreaker)(nil)
-
 // apply implements Option.
 func (s *SlidingWindowBreaker) apply(o *options) error {
 	if o.halfOpenDelay == 0 || o.halfOpenDelay > s.windowSize {

hoglet.go

@@ -42,6 +42,8 @@ type Breaker interface {
 	// The halfOpen parameter indicates whether the call was made in half-open state.
 	// The failure parameter indicates whether the call failed.
 	observe(halfOpen, failure bool) stateChange
+
+	Option // breakers can also modify or sanity-check their circuit's options
 }
 
 // ObserverFactory is an interface that allows customizing the per-call observer creation.
@@ -87,6 +89,8 @@ func NewCircuit[IN, OUT any](f WrappedFunc[IN, OUT], breaker Breaker, opts ...Op
 
 	if breaker != nil {
 		o.breaker = breaker
+		// apply breaker as last, so it can verify
+		opts = append(opts, breaker)
 	}
 
 	// the default observerFactory is the circuit itself
@@ -98,18 +102,8 @@ func NewCircuit[IN, OUT any](f WrappedFunc[IN, OUT], breaker Breaker, opts ...Op
 		}
 	}
 
-	if breakerOpt, ok := breaker.(Option); ok {
-		if err := breakerOpt.apply(&o); err != nil {
-			return nil, fmt.Errorf("applying breaker option: %w", err)
-		}
-	}
-
 	c.options = o
 
-	if _, ok := breaker.(*EWMABreaker); ok && c.halfOpenDelay == 0 {
-		return nil, fmt.Errorf("EWMABreaker requires a half-open delay")
-	}
-
 	return c, nil
 }
 
@@ -297,3 +291,7 @@ type noopBreaker struct{}
 func (noopBreaker) observe(halfOpen, failure bool) stateChange {
 	return stateChangeNone
 }
+
+func (noopBreaker) apply(*options) error {
+	return nil
+}

hoglet_test.go

@@ -122,6 +122,10 @@ func (mt *mockBreaker) observe(halfOpen, failure bool) stateChange {
 	return stateChangeClose
 }
 
+func (mt *mockBreaker) apply(o *options) error {
+	return nil
+}
+
 func TestHoglet_Do(t *testing.T) {
 	type calls struct {
 		arg       noopIn