func testFunction(t *testing.T, config testFunctionConfig) {
	for i, tc := range config.Cases {
		ast, err := lang.Parse(tc.Input)
		if err != nil {
			t.Fatalf("%d: err: %s", i, err)
		}

		out, _, err := lang.Eval(ast, langEvalConfig(config.Vars))
		if (err != nil) != tc.Error {
			t.Fatalf("%d: err: %s", i, err)
		}

		if !reflect.DeepEqual(out, tc.Result) {
			t.Fatalf(
				"%d: bad output for input: %s\n\nOutput: %#v\nExpected: %#v",
				i, tc.Input, out, tc.Result)
		}
	}
}
// Interpolate uses the given mapping of variable values and uses
// those as the values to replace any variables in this raw
// configuration.
//
// Any prior calls to Interpolate are replaced with this one.
//
// If a variable key is missing, this will panic.
func (r *RawConfig) Interpolate(vs map[string]ast.Variable) error {
	r.lock.Lock()
	defer r.lock.Unlock()

	config := langEvalConfig(vs)
	return r.interpolate(func(root ast.Node) (string, error) {
		// We detect the variables again and check if the value of any
		// of the variables is the computed value. If it is, then we
		// treat this entire value as computed.
		//
		// We have to do this here before the `lang.Eval` because
		// if any of the variables it depends on are computed, then
		// the interpolation can fail at runtime for other reasons. Example:
		// `${count.index+1}`: in a world where `count.index` is computed,
		// this would fail a type check since the computed placeholder is
		// a string, but realistically the whole value is just computed.
		vars, err := DetectVariables(root)
		if err != nil {
			return "", err
		}
		for _, v := range vars {
			varVal, ok := vs[v.FullKey()]
			if ok && varVal.Value == UnknownVariableValue {
				return UnknownVariableValue, nil
			}
		}

		// None of the variables we need are computed, meaning we should
		// be able to properly evaluate.
		out, _, err := lang.Eval(root, config)
		if err != nil {
			return "", err
		}

		return out.(string), nil
	})
}
// Validate does some basic semantic checking of the configuration.
func (c *Config) Validate() error {
	if c == nil {
		return nil
	}

	var errs []error

	for _, k := range c.unknownKeys {
		errs = append(errs, fmt.Errorf(
			"Unknown root level key: %s", k))
	}

	vars := c.InterpolatedVariables()
	varMap := make(map[string]*Variable)
	for _, v := range c.Variables {
		varMap[v.Name] = v
	}

	for _, v := range c.Variables {
		if v.Type() == VariableTypeUnknown {
			errs = append(errs, fmt.Errorf(
				"Variable '%s': must be string or mapping",
				v.Name))
			continue
		}

		interp := false
		fn := func(ast.Node) (string, error) {
			interp = true
			return "", nil
		}

		w := &interpolationWalker{F: fn}
		if v.Default != nil {
			if err := reflectwalk.Walk(v.Default, w); err == nil {
				if interp {
					errs = append(errs, fmt.Errorf(
						"Variable '%s': cannot contain interpolations",
						v.Name))
				}
			}
		}
	}

	// Check for references to user variables that do not actually
	// exist and record those errors.
	for source, vs := range vars {
		for _, v := range vs {
			uv, ok := v.(*UserVariable)
			if !ok {
				continue
			}

			if _, ok := varMap[uv.Name]; !ok {
				errs = append(errs, fmt.Errorf(
					"%s: unknown variable referenced: '%s'. define it with 'variable' blocks",
					source,
					uv.Name))
			}
		}
	}

	// Check that all count variables are valid.
	for source, vs := range vars {
		for _, rawV := range vs {
			switch v := rawV.(type) {
			case *CountVariable:
				if v.Type == CountValueInvalid {
					errs = append(errs, fmt.Errorf(
						"%s: invalid count variable: %s",
						source,
						v.FullKey()))
				}
			case *PathVariable:
				if v.Type == PathValueInvalid {
					errs = append(errs, fmt.Errorf(
						"%s: invalid path variable: %s",
						source,
						v.FullKey()))
				}
			}
		}
	}

	// Check that providers aren't declared multiple times.
	providerSet := make(map[string]struct{})
	for _, p := range c.ProviderConfigs {
		name := p.FullName()
		if _, ok := providerSet[name]; ok {
			errs = append(errs, fmt.Errorf(
				"provider.%s: declared multiple times, you can only declare a provider once",
				name))
			continue
		}

		providerSet[name] = struct{}{}
	}

	// Check that all references to modules are valid
	modules := make(map[string]*Module)
	dupped := make(map[string]struct{})
	for _, m := range c.Modules {
		// Check for duplicates
		if _, ok := modules[m.Id()]; ok {
			if _, ok := dupped[m.Id()]; !ok {
				dupped[m.Id()] = struct{}{}

				errs = append(errs, fmt.Errorf(
					"%s: module repeated multiple times",
					m.Id()))
			}

			// Already seen this module, just skip it
			continue
		}

		modules[m.Id()] = m

		// Check that the source has no interpolations
		rc, err := NewRawConfig(map[string]interface{}{
			"root": m.Source,
		})
		if err != nil {
			errs = append(errs, fmt.Errorf(
				"%s: module source error: %s",
				m.Id(), err))
		} else if len(rc.Interpolations) > 0 {
			errs = append(errs, fmt.Errorf(
				"%s: module source cannot contain interpolations",
				m.Id()))
		}

		// Check that the name matches our regexp
		if !NameRegexp.Match([]byte(m.Name)) {
			errs = append(errs, fmt.Errorf(
				"%s: module name can only contain letters, numbers, "+
					"dashes, and underscores",
				m.Id()))
		}

		// Check that the configuration can all be strings
		raw := make(map[string]interface{})
		for k, v := range m.RawConfig.Raw {
			var strVal string
			if err := mapstructure.WeakDecode(v, &strVal); err != nil {
				errs = append(errs, fmt.Errorf(
					"%s: variable %s must be a string value",
					m.Id(), k))
			}
			raw[k] = strVal
		}

		// Check for invalid count variables
		for _, v := range m.RawConfig.Variables {
			switch v.(type) {
			case *CountVariable:
				errs = append(errs, fmt.Errorf(
					"%s: count variables are only valid within resources", m.Name))
			case *SelfVariable:
				errs = append(errs, fmt.Errorf(
					"%s: self variables are only valid within resources", m.Name))
			}
		}

		// Update the raw configuration to only contain the string values
		m.RawConfig, err = NewRawConfig(raw)
		if err != nil {
			errs = append(errs, fmt.Errorf(
				"%s: can't initialize configuration: %s",
				m.Id(), err))
		}
	}
	dupped = nil

	// Check that all variables for modules reference modules that
	// exist.
	for source, vs := range vars {
		for _, v := range vs {
			mv, ok := v.(*ModuleVariable)
			if !ok {
				continue
			}

			if _, ok := modules[mv.Name]; !ok {
				errs = append(errs, fmt.Errorf(
					"%s: unknown module referenced: %s",
					source,
					mv.Name))
			}
		}
	}

	// Check that all references to resources are valid
	resources := make(map[string]*Resource)
	dupped = make(map[string]struct{})
	for _, r := range c.Resources {
		if _, ok := resources[r.Id()]; ok {
			if _, ok := dupped[r.Id()]; !ok {
				dupped[r.Id()] = struct{}{}

				errs = append(errs, fmt.Errorf(
					"%s: resource repeated multiple times",
					r.Id()))
			}
		}

		resources[r.Id()] = r
	}
	dupped = nil

	// Validate resources
	for n, r := range resources {
		// Verify count variables
		for _, v := range r.RawCount.Variables {
			switch v.(type) {
			case *CountVariable:
				errs = append(errs, fmt.Errorf(
					"%s: resource count can't reference count variable: %s",
					n,
					v.FullKey()))
			case *ModuleVariable:
				errs = append(errs, fmt.Errorf(
					"%s: resource count can't reference module variable: %s",
					n,
					v.FullKey()))
			case *ResourceVariable:
				errs = append(errs, fmt.Errorf(
					"%s: resource count can't reference resource variable: %s",
					n,
					v.FullKey()))
			case *UserVariable:
				// Good
			default:
				panic("Unknown type in count var: " + n)
			}
		}

		// Interpolate with a fixed number to verify that its a number.
		r.RawCount.interpolate(func(root ast.Node) (string, error) {
			// Execute the node but transform the AST so that it returns
			// a fixed value of "5" for all interpolations.
			out, _, err := lang.Eval(
				lang.FixedValueTransform(
					root, &ast.LiteralNode{Value: "5", Typex: ast.TypeString}),
				nil)
			if err != nil {
				return "", err
			}

			return out.(string), nil
		})
		_, err := strconv.ParseInt(r.RawCount.Value().(string), 0, 0)
		if err != nil {
			errs = append(errs, fmt.Errorf(
				"%s: resource count must be an integer",
				n))
		}
		r.RawCount.init()

		// Verify depends on points to resources that all exist
		for _, d := range r.DependsOn {
			// Check if we contain interpolations
			rc, err := NewRawConfig(map[string]interface{}{
				"value": d,
			})
			if err == nil && len(rc.Variables) > 0 {
				errs = append(errs, fmt.Errorf(
					"%s: depends on value cannot contain interpolations: %s",
					n, d))
				continue
			}

			if _, ok := resources[d]; !ok {
				errs = append(errs, fmt.Errorf(
					"%s: resource depends on non-existent resource '%s'",
					n, d))
			}
		}

		// Verify provider points to a provider that is configured
		if r.Provider != "" {
			if _, ok := providerSet[r.Provider]; !ok {
				errs = append(errs, fmt.Errorf(
					"%s: resource depends on non-configured provider '%s'",
					n, r.Provider))
			}
		}

		// Verify provisioners don't contain any splats
		for _, p := range r.Provisioners {
			// This validation checks that there are now splat variables
			// referencing ourself. This currently is not allowed.

			for _, v := range p.ConnInfo.Variables {
				rv, ok := v.(*ResourceVariable)
				if !ok {
					continue
				}

				if rv.Multi && rv.Index == -1 && rv.Type == r.Type && rv.Name == r.Name {
					errs = append(errs, fmt.Errorf(
						"%s: connection info cannot contain splat variable "+
							"referencing itself", n))
					break
				}
			}

			for _, v := range p.RawConfig.Variables {
				rv, ok := v.(*ResourceVariable)
				if !ok {
					continue
				}

				if rv.Multi && rv.Index == -1 && rv.Type == r.Type && rv.Name == r.Name {
					errs = append(errs, fmt.Errorf(
						"%s: connection info cannot contain splat variable "+
							"referencing itself", n))
					break
				}
			}
		}
	}

	for source, vs := range vars {
		for _, v := range vs {
			rv, ok := v.(*ResourceVariable)
			if !ok {
				continue
			}

			id := fmt.Sprintf("%s.%s", rv.Type, rv.Name)
			if _, ok := resources[id]; !ok {
				errs = append(errs, fmt.Errorf(
					"%s: unknown resource '%s' referenced in variable %s",
					source,
					id,
					rv.FullKey()))
				continue
			}
		}
	}

	// Check that all outputs are valid
	for _, o := range c.Outputs {
		invalid := false
		for k, _ := range o.RawConfig.Raw {
			if k != "value" {
				invalid = true
				break
			}
		}
		if invalid {
			errs = append(errs, fmt.Errorf(
				"%s: output should only have 'value' field", o.Name))
		}

		for _, v := range o.RawConfig.Variables {
			if _, ok := v.(*CountVariable); ok {
				errs = append(errs, fmt.Errorf(
					"%s: count variables are only valid within resources", o.Name))
			}
		}
	}

	// Check that all variables are in the proper context
	for source, rc := range c.rawConfigs() {
		walker := &interpolationWalker{
			ContextF: c.validateVarContextFn(source, &errs),
		}
		if err := reflectwalk.Walk(rc.Raw, walker); err != nil {
			errs = append(errs, fmt.Errorf(
				"%s: error reading config: %s", source, err))
		}
	}

	// Validate the self variable
	for source, rc := range c.rawConfigs() {
		// Ignore provisioners. This is a pretty brittle way to do this,
		// but better than also repeating all the resources.
		if strings.Contains(source, "provision") {
			continue
		}

		for _, v := range rc.Variables {
			if _, ok := v.(*SelfVariable); ok {
				errs = append(errs, fmt.Errorf(
					"%s: cannot contain self-reference %s", source, v.FullKey()))
			}
		}
	}

	if len(errs) > 0 {
		return &multierror.Error{Errors: errs}
	}

	return nil
}