Add support for type-safe Start* function (#468)

* add HandlerFunc interface and StartWithOptionsTypeSafe

* add tests for StartWithOptionsTypeSafe

* Update test to cover bool from validateArguments

* move generic code to dedicated files with build tag

* Reduce HandlerFunc interface to just one variant. Rename StartWithOptionsTypeSafe to StartHandlerFunc

* add ValidateHandlerFunc

* Revert "add ValidateHandlerFunc"

This reverts commit bc1e02e.

* add context-arg-only reflections back to the godoc

* update copyright date in new file to match year of file's creation

* update copyright year in new test file to match the year of file's creation

Co-authored-by: Bryan Moffatt <bmoffatt@users.noreply.github.com>

Author: logandavies181

lambda/entry.go

@@ -17,20 +17,23 @@ import (
 //   - handler must be a function
 //   - handler may take between 0 and two arguments.
 //   - if there are two arguments, the first argument must satisfy the "context.Context" interface.
-//   - handler may return between 0 and two arguments.
-//   - if there are two return values, the second argument must be an error.
+//   - handler may return between 0 and two values.
+//   - if there are two return values, the second return value must be an error.
 //   - if there is one return value it must be an error.
 //
 // Valid function signatures:
 //
 //	func ()
+//	func (TIn)
 //	func () error
 //	func (TIn) error
 //	func () (TOut, error)
 //	func (TIn) (TOut, error)
+//	func (context.Context)
 //	func (context.Context) error
-//	func (context.Context, TIn) error
 //	func (context.Context) (TOut, error)
+//	func (context.Context, TIn)
+//	func (context.Context, TIn) error
 //	func (context.Context, TIn) (TOut, error)
 //
 // Where "TIn" and "TOut" are types compatible with the "encoding/json" standard library.

lambda/entry_generic.go

@@ -0,0 +1,23 @@
+//go:build go1.18
+// +build go1.18
+
+// Copyright 2022 Amazon.com, Inc. or its affiliates. All Rights Reserved
+
+package lambda
+
+import (
+	"context"
+)
+
+// HandlerFunc represents a valid input with two arguments and two returns as described by Start
+type HandlerFunc[TIn, TOut any] interface {
+	func(context.Context, TIn) (TOut, error)
+}
+
+// StartHandlerFunc is the same as StartWithOptions except that it takes a generic input
+// so that the function signature can be validated at compile time.
+//
+// Currently only the `func (context.Context, TIn) (TOut, error)` variant is supported
+func StartHandlerFunc[TIn any, TOut any, H HandlerFunc[TIn, TOut]](handler H, options ...Option) {
+	start(newHandler(handler, options...))
+}

lambda/entry_generic_test.go

@@ -0,0 +1,36 @@
+//go:build go1.18
+// +build go1.18
+
+// Copyright 2022 Amazon.com, Inc. or its affiliates. All Rights Reserved
+
+package lambda
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStartHandlerFunc(t *testing.T) {
+	actual := "unexpected"
+	logFatalf = func(format string, v ...interface{}) {
+		actual = fmt.Sprintf(format, v...)
+	}
+
+	f := func(context.Context, any) (any, error) { return 1, nil }
+	StartHandlerFunc(f)
+
+	assert.Equal(t, "expected AWS Lambda environment variables [_LAMBDA_SERVER_PORT AWS_LAMBDA_RUNTIME_API] are not defined", actual)
+
+	handlerType := reflect.TypeOf(f)
+
+	handlerTakesContext, err := validateArguments(handlerType)
+	assert.NoError(t, err)
+	assert.True(t, handlerTakesContext)
+
+	err = validateReturns(handlerType)
+	assert.NoError(t, err)
+}