hcldec: RefineValueSpec

This new spec type allows adding value refinements to the results of some
other spec, as long as the wrapped spec does indeed enforce the
constraints described by the refinements.

hcldec/spec.go

@@ -1606,7 +1606,52 @@ func (s *TransformFuncSpec) sourceRange(content *hcl.BodyContent, blockLabels []
 	return s.Wrapped.sourceRange(content, blockLabels)
 }
 
-// ValidateFuncSpec is a spec that allows for extended
+// RefineValueSpec is a spec that wraps another and applies a fixed set of [cty]
+// value refinements to whatever value it produces.
+//
+// Refinements serve to constrain the range of any unknown values, and act as
+// assertions for known values by panicking if the final value does not meet
+// the refinement. Therefore applications using this spec must guarantee that
+// any value passing through the RefineValueSpec will always be consistent with
+// the refinements; if not then that is a bug in the application.
+//
+// The wrapped spec should typically be a [ValidateSpec], a [TransformFuncSpec],
+// or some other adapter that guarantees that the inner result cannot possibly
+// violate the refinements.
+type RefineValueSpec struct {
+	Wrapped Spec
+
+	// Refine is a function which accepts a builder for a refinement in
+	// progress and uses the builder pattern to add extra refinements to it,
+	// finally returning the same builder with those modifications applied.
+	Refine func(*cty.RefinementBuilder) *cty.RefinementBuilder
+}
+
+func (s *RefineValueSpec) visitSameBodyChildren(cb visitFunc) {
+	cb(s.Wrapped)
+}
+
+func (s *RefineValueSpec) decode(content *hcl.BodyContent, blockLabels []blockLabel, ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
+	wrappedVal, diags := s.Wrapped.decode(content, blockLabels, ctx)
+	if diags.HasErrors() {
+		// We won't try to run our function in this case, because it'll probably
+		// generate confusing additional errors that will distract from the
+		// root cause.
+		return cty.UnknownVal(s.impliedType()), diags
+	}
+
+	return wrappedVal.RefineWith(s.Refine), diags
+}
+
+func (s *RefineValueSpec) impliedType() cty.Type {
+	return s.Wrapped.impliedType()
+}
+
+func (s *RefineValueSpec) sourceRange(content *hcl.BodyContent, blockLabels []blockLabel) hcl.Range {
+	return s.Wrapped.sourceRange(content, blockLabels)
+}
+
+// ValidateSpec is a spec that allows for extended
 // developer-defined validation. The validation function receives the
 // result of the wrapped spec.
 //

hcldec/spec_test.go

@@ -9,6 +9,8 @@ import (
 	"testing"
 
 	"github.com/apparentlymart/go-dump/dump"
+	"github.com/google/go-cmp/cmp"
+	"github.com/zclconf/go-cty-debug/ctydebug"
 	"github.com/zclconf/go-cty/cty"
 
 	"github.com/hashicorp/hcl/v2"
@@ -210,3 +212,72 @@ foo = "invalid"
 		})
 	}
 }
+
+func TestRefineValueSpec(t *testing.T) {
+	config := `
+foo = "hello"
+bar = unk
+`
+
+	f, diags := hclsyntax.ParseConfig([]byte(config), "", hcl.InitialPos)
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
+
+	attrSpec := func(name string) Spec {
+		return &RefineValueSpec{
+			// RefineValueSpec should typically have a ValidateSpec wrapped
+			// inside it to catch any values that are outside of the required
+			// range and return a helpful error message about it. In this
+			// case our refinement is .NotNull so the validation function
+			// must reject null values.
+			Wrapped: &ValidateSpec{
+				Wrapped: &AttrSpec{
+					Name:     name,
+					Required: true,
+					Type:     cty.String,
+				},
+				Func: func(value cty.Value) hcl.Diagnostics {
+					var diags hcl.Diagnostics
+					if value.IsNull() {
+						diags = diags.Append(&hcl.Diagnostic{
+							Severity: hcl.DiagError,
+							Summary:  "Cannot be null",
+							Detail:   "Argument is required.",
+						})
+					}
+					return diags
+				},
+			},
+			Refine: func(rb *cty.RefinementBuilder) *cty.RefinementBuilder {
+				return rb.NotNull()
+			},
+		}
+	}
+	spec := &ObjectSpec{
+		"foo": attrSpec("foo"),
+		"bar": attrSpec("bar"),
+	}
+
+	got, diags := Decode(f.Body, spec, &hcl.EvalContext{
+		Variables: map[string]cty.Value{
+			"unk": cty.UnknownVal(cty.String),
+		},
+	})
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
+
+	want := cty.ObjectVal(map[string]cty.Value{
+		// This argument had a known value, so it's unchanged but the
+		// RefineValueSpec still checks that it isn't null to catch
+		// bugs in the application's validation function.
+		"foo": cty.StringVal("hello"),
+
+		// The final value of bar is unknown but refined as non-null.
+		"bar": cty.UnknownVal(cty.String).RefineNotNull(),
+	})
+	if diff := cmp.Diff(want, got, ctydebug.CmpOptions); diff != "" {
+		t.Errorf("wrong result\n%s", diff)
+	}
+}