Second revision.

Split paths and the wildcard version out into two different
abstractions. Move them into the `attr` package.

Author: Paddy Carver

attr/path.go

@@ -0,0 +1,107 @@
+package attr
+
+// Path is used to identify part of a tfsdk.Schema, attr.Type, or attr.Value.
+// It is a series of steps, describing a traversal of a Terraform type. Each
+// Terraform type has its own limits on what steps can be used to traverse it.
+type Path struct {
+	steps []step
+}
+
+// New returns an Path that is ready to be used.
+func NewPath() Path {
+	return Path{}
+}
+
+func copySteps(in []step) []step {
+	out := make([]step, len(in))
+	copy(out, in)
+	return out
+}
+
+// IsEmpty returns true if a Path has no steps and effectively points to the
+// root of the value.
+func (a Path) IsEmpty() bool {
+	return len(a.steps) < 1
+}
+
+// Parent returns a Path pointing to the parent of the attribute or element
+// that `a` points to. If `a` has no parent, an empty Path is returned.
+func (a Path) Parent() Path {
+	if len(a.steps) < 1 {
+		return Path{steps: []step{}}
+	}
+	return Path{
+		steps: copySteps(a.steps)[:len(a.steps)-1],
+	}
+}
+
+// Attribute returns a copy of `a`, with another step added to select the named
+// attribute of the object that `a` points to.
+func (a Path) Attribute(name string) Path {
+	return Path{
+		steps: append(copySteps(a.steps), attributeName(name)),
+	}
+}
+
+// ElementKey returns a copy of `a` with another step added to select the named
+// key of the map that `a` points to.
+func (a Path) ElementKey(name string) Path {
+	return Path{
+		steps: append(copySteps(a.steps), elementKeyString(name)),
+	}
+}
+
+// ElementPos returns a copy of `a` with another step added to select the
+// element that is in the specified position of the list or tuple that `a`
+// points to.
+func (a Path) ElementPos(pos int) Path {
+	return Path{
+		steps: append(copySteps(a.steps), elementKeyInt(pos)),
+	}
+}
+
+// Element returns a copy of `a` with another step added to select the
+// specified element of the set that `a` points to.
+func (a Path) Element(val Value) Path {
+	return Path{
+		steps: append(copySteps(a.steps), elementKeyValue{Value: val}),
+	}
+}
+
+type step interface {
+	unimplementablePathStep()
+}
+
+// attributeName is a step that selects a single attribute, by its name. It is
+// used on attr.TypeWithAttributeTypes, tfsdk.NestedAttributes, and
+// tfsdk.Schema types.
+type attributeName string
+
+func (a attributeName) unimplementablePathStep() {}
+
+// elementKeyString is a step that selects a single element using a string
+// index. It is used on attr.TypeWithElementType types that return a
+// tftypes.Map from their TerraformType method. It is also used on
+// tfsdk.MapNestedAttribute nested attributes.
+type elementKeyString string
+
+func (e elementKeyString) unimplementablePathStep() {}
+
+// elementKeyInt is a step that selects a single element using an integer
+// index. It is used on attr.TypeWithElementTypes types and
+// attr.TypeWithElementType types that return a tftypes.List from their
+// TerraformType method. It is also used on tfsdk.ListNestedAttribute nested
+// attributes.
+type elementKeyInt int
+
+func (e elementKeyInt) unimplementablePathStep() {}
+
+// elementKeyValue is a step that selects a single element using an attr.Value
+// index. It is used on attr.TypeWithElementType types that return a
+// tftypes.Set from their TerraformType method. It is also used on
+// tfsdk.SetNestedAttribute nested attributes.
+type elementKeyValue struct {
+	Value Value
+}
+
+func (e elementKeyValue) unimplementablePathStep() {}