chore: add HandlerFnOfOK to provide canonical request body validation

sdk.go

@@ -46,6 +46,18 @@ func HandleFnOf[T any](fn func(context.Context, RequestOf[T]) Response) Handler
 	})
 }
 
+// HandlerFnOfOK provides a means to translate the incoming requests to the destination body type
+// and execute validation on that type. This normalizes the sad path for both the unmarshalling of
+// the request body and the validation of that request type using its OK() method.
+func HandlerFnOfOK[T interface{ OK() []APIError }](fn func(context.Context, RequestOf[T]) Response) Handler {
+	return HandleFnOf(func(ctx context.Context, r RequestOf[T]) Response {
+		if errs := r.Body.OK(); len(errs) > 0 {
+			return ErrResp(errs...)
+		}
+		return fn(ctx, r)
+	})
+}
+
 // WorkflowCtx is the Request.Context field when integrating a function with Falcon Fusion workflow.
 type WorkflowCtx struct {
 	ActivityExecID    string `json:"activity_execution_id"`

sdk_test.go

@@ -1,6 +1,8 @@
 package fdk_test
 
 import (
+	"context"
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"testing"
@@ -19,3 +21,46 @@ func TestAPIError(t *testing.T) {
 		fdk.EqualVals(t, want, err.Error())
 	}
 }
+
+func TestHandlerFnOfOK(t *testing.T) {
+	h := fdk.HandlerFnOfOK(func(ctx context.Context, r fdk.RequestOf[okReq]) fdk.Response {
+		return fdk.Response{Code: http.StatusOK}
+	})
+
+	b, err := json.Marshal(okReq{
+		Shoulds: []int{
+			http.StatusOK,                  // should not cause an error
+			http.StatusBadRequest,          // should return a 400 error
+			http.StatusInternalServerError, // should return a 500 error
+		},
+	})
+	mustNoErr(t, err)
+
+	resp := h.Handle(context.TODO(), fdk.Request{Body: b})
+	fdk.EqualVals(t, http.StatusInternalServerError, resp.Code)
+
+	wantErrs := []fdk.APIError{
+		{Code: http.StatusBadRequest, Message: http.StatusText(http.StatusBadRequest)},
+		{Code: http.StatusInternalServerError, Message: http.StatusText(http.StatusInternalServerError)},
+	}
+	if !fdk.EqualVals(t, len(wantErrs), len(resp.Errors)) {
+		return
+	}
+	fdk.EqualVals(t, wantErrs[0], resp.Errors[0])
+	fdk.EqualVals(t, wantErrs[1], resp.Errors[1])
+}
+
+type okReq struct {
+	Shoulds []int `json:"shoulds"`
+}
+
+func (r okReq) OK() []fdk.APIError {
+	var out []fdk.APIError
+	for _, sh := range r.Shoulds {
+		if sh < 400 {
+			continue
+		}
+		out = append(out, fdk.APIError{Code: sh, Message: http.StatusText(sh)})
+	}
+	return out
+}