more experimenting with generics

This approach might be workable, but I don't see how just yet

Author: Sean Sorrell

types/directives.go

@@ -0,0 +1,61 @@
+/**
+*
+* Experimental code with generics
+*
+**/
+package types
+
+import (
+	"context"
+)
+
+// we must express this type as an interface that the Go type checker can assert on
+// this duplicates code elsewhere in the `types` package
+type InputValue interface {
+	string | int32
+}
+
+type OutputValue interface {
+	string
+}
+
+// generic interface with type parameter
+type BeforeVisitor[In InputValue] interface {
+	Before(context.Context, Directive, In) error
+}
+
+type AfterVisitor[Out OutputValue] interface {
+	After(context.Context, Directive, Out) (Out, error)
+}
+
+type DirectiveVisitorI[In InputValue, Out OutputValue] interface {
+	BeforeVisitor[In]
+	AfterVisitor[Out]
+}
+
+type customDirectiveImpl struct {
+}
+
+// OK: compiles
+var _ DirectiveVisitorI[string,string] = &customDirectiveImpl{}
+
+// error: "any" does not implement InputValue
+var _ DirectiveVisitorI[any,any] = &customDirectiveImpl{}
+
+// error: cannot use type InputValue outside of type constraint, contains type constraints
+// 
+// error: *customDirectiveImpl does not implement DirectiveVisitorI[InputValue, OutputValue] (wrong type for After method)
+// have After(ctx context.Context, dir Directive, _ string) (string, error)
+// want After(context.Context, Directive, OutputValue) (OutputValue, error) (go-build)
+// 
+var _ DirectiveVisitorI[InputValue, OutputValue] = &customDirectiveImpl{}
+
+func (v *customDirectiveImpl) Before(ctx context.Context, dir Directive, _ string) error {
+	return nil
+}
+
+func (v *customDirectiveImpl) After(ctx context.Context, dir Directive, _ string) (string, error) {
+	return "", nil
+}
+
+