[Go] return an error if a nil Model, Embedder, etc is used. (#613)

go/ai/embedder.go

@@ -16,6 +16,7 @@ package ai
 
 import (
 	"context"
+	"errors"
 
 	"github.com/firebase/genkit/go/core"
 	"github.com/firebase/genkit/go/internal/atype"
@@ -61,6 +62,9 @@ func LookupEmbedder(provider, name string) *Embedder {
 
 // Embed runs the given [Embedder].
 func (e *Embedder) Embed(ctx context.Context, req *EmbedRequest) (*EmbedResponse, error) {
+	if e == nil {
+		return nil, errors.New("Embed called on a nil Embedder; check that all embedders are defined")
+	}
 	a := (*core.Action[*EmbedRequest, *EmbedResponse, struct{}])(e)
 	return a.Run(ctx, req, nil)
 }

go/ai/generate.go

@@ -83,6 +83,9 @@ func LookupModel(provider, name string) *Model {
 
 // Generate applies the [Model] to some input, handling tool requests.
 func (m *Model) Generate(ctx context.Context, req *GenerateRequest, cb ModelStreamingCallback) (*GenerateResponse, error) {
+	if m == nil {
+		return nil, errors.New("Generate called on a nil Model; check that all models are defined")
+	}
 	if err := conformOutput(req); err != nil {
 		return nil, err
 	}

go/ai/prompt.go

@@ -16,6 +16,7 @@ package ai
 
 import (
 	"context"
+	"errors"
 	"maps"
 
 	"github.com/firebase/genkit/go/core"
@@ -49,5 +50,8 @@ func LookupPrompt(provider, name string) *Prompt {
 
 // Render renders the [Prompt] with some input data.
 func (p *Prompt) Render(ctx context.Context, input any) (*GenerateRequest, error) {
+	if p == nil {
+		return nil, errors.New("Render called on a nil Prompt; check that all prompts are defined")
+	}
 	return (*core.Action[any, *GenerateRequest, struct{}])(p).Run(ctx, input, nil)
 }

go/ai/retriever.go

@@ -16,6 +16,7 @@ package ai
 
 import (
 	"context"
+	"errors"
 
 	"github.com/firebase/genkit/go/core"
 	"github.com/firebase/genkit/go/internal/atype"
@@ -81,12 +82,18 @@ func LookupRetriever(provider, name string) *Retriever {
 
 // Index runs the given [Indexer].
 func (i *Indexer) Index(ctx context.Context, req *IndexerRequest) error {
+	if i == nil {
+		return errors.New("Index called on a nil Indexer; check that all indexers are defined")
+	}
 	_, err := (*indexerAction)(i).Run(ctx, req, nil)
 	return err
 }
 
 // Retrieve runs the given [Retriever].
 func (r *Retriever) Retrieve(ctx context.Context, req *RetrieverRequest) (*RetrieverResponse, error) {
+	if r == nil {
+		return nil, errors.New("Retriever called on a nil Retriever; check that all retrievers are defined")
+	}
 	return (*retrieverAction)(r).Run(ctx, req, nil)
 }