bake: allow user functions in variables and vice-versa

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

bake/bake.go

@@ -406,6 +406,13 @@ type Variable struct {
 	Default *hcl.Attribute `json:"default,omitempty" hcl:"default,optional"`
 }
 
+type Function struct {
+	Name     string         `json:"-" hcl:"name,label"`
+	Params   *hcl.Attribute `json:"params,omitempty" hcl:"params"`
+	Variadic *hcl.Attribute `json:"variadic_param,omitempty" hcl:"variadic_params"`
+	Result   *hcl.Attribute `json:"result,omitempty" hcl:"result"`
+}
+
 type Group struct {
 	Name    string   `json:"-" hcl:"name,label"`
 	Targets []string `json:"targets" hcl:"targets"`

bake/hcl.go

@@ -4,8 +4,10 @@ import (
 	"math"
 	"math/big"
 	"os"
+	"reflect"
 	"strconv"
 	"strings"
+	"unsafe"
 
 	"github.com/docker/buildx/util/userfunc"
 	"github.com/hashicorp/go-cty-funcs/cidr"
@@ -131,14 +133,18 @@ var (
 
 type StaticConfig struct {
 	Variables []*Variable `hcl:"variable,block"`
+	Functions []*Function `hcl:"function,block"`
 	Remain    hcl.Body    `hcl:",remain"`
 
 	attrs hcl.Attributes
 
 	defaults  map[string]*hcl.Attribute
+	funcDefs  map[string]*Function
+	funcs     map[string]function.Function
 	env       map[string]string
-	values   map[string]cty.Value
+	ectx      hcl.EvalContext
 	progress  map[string]struct{}
+	progressF map[string]struct{}
 }
 
 func mergeStaticConfig(scs []*StaticConfig) *StaticConfig {
@@ -148,6 +154,7 @@ func mergeStaticConfig(scs []*StaticConfig) *StaticConfig {
 	sc := scs[0]
 	for _, s := range scs[1:] {
 		sc.Variables = append(sc.Variables, s.Variables...)
+		sc.Functions = append(sc.Functions, s.Functions...)
 		for k, v := range s.attrs {
 			sc.attrs[k] = v
 		}
@@ -155,9 +162,16 @@ func mergeStaticConfig(scs []*StaticConfig) *StaticConfig {
 	return sc
 }
 
-func (sc *StaticConfig) Values(withEnv bool) (map[string]cty.Value, error) {
+func (sc *StaticConfig) EvalContext(withEnv bool) (*hcl.EvalContext, error) {
+	// json parser also parses blocks as attributes
+	delete(sc.attrs, "target")
+	delete(sc.attrs, "function")
+
 	sc.defaults = map[string]*hcl.Attribute{}
 	for _, v := range sc.Variables {
+		if v.Name == "target" {
+			continue
+		}
 		sc.defaults[v.Name] = v.Default
 	}
 
@@ -171,35 +185,305 @@ func (sc *StaticConfig) Values(withEnv bool) (map[string]cty.Value, error) {
 		}
 	}
 
-	sc.values = map[string]cty.Value{}
-	sc.progress = map[string]struct{}{}
+	sc.funcDefs = map[string]*Function{}
+	for _, v := range sc.Functions {
+		sc.funcDefs[v.Name] = v
+	}
 
+	sc.ectx = hcl.EvalContext{
+		Variables: map[string]cty.Value{},
+		Functions: stdlibFunctions,
+	}
+	sc.funcs = map[string]function.Function{}
+	sc.progress = map[string]struct{}{}
+	sc.progressF = map[string]struct{}{}
 	for k := range sc.attrs {
-		if _, err := sc.resolveValue(k); err != nil {
+		if err := sc.resolveValue(k); err != nil {
 			return nil, err
 		}
 	}
 
 	for k := range sc.defaults {
-		if _, err := sc.resolveValue(k); err != nil {
+		if err := sc.resolveValue(k); err != nil {
 			return nil, err
 		}
 	}
-	return sc.values, nil
+
+	for k := range sc.funcDefs {
+		if err := sc.resolveFunction(k); err != nil {
+			return nil, err
+		}
+	}
+	return &sc.ectx, nil
 }
 
-func (sc *StaticConfig) resolveValue(name string) (v *cty.Value, err error) {
-	if v, ok := sc.values[name]; ok {
-		return &v, nil
+type jsonExp interface {
+	ExprList() []hcl.Expression
+	ExprMap() []hcl.KeyValuePair
+}
+
+func elementExpressions(je jsonExp, exp hcl.Expression) []hcl.Expression {
+	list := je.ExprList()
+	if len(list) != 0 {
+		exp := make([]hcl.Expression, 0, len(list))
+		for _, e := range list {
+			if je, ok := e.(jsonExp); ok {
+				exp = append(exp, elementExpressions(je, e)...)
+			}
+		}
+		return exp
+	}
+	kvlist := je.ExprMap()
+	if len(kvlist) != 0 {
+		exp := make([]hcl.Expression, 0, len(kvlist)*2)
+		for _, p := range kvlist {
+			exp = append(exp, p.Key)
+			if je, ok := p.Value.(jsonExp); ok {
+				exp = append(exp, elementExpressions(je, p.Value)...)
+			}
+		}
+		return exp
+	}
+	return []hcl.Expression{exp}
+}
+
+func jsonFuncCallsRecursive(exp hcl.Expression) ([]string, error) {
+	je, ok := exp.(jsonExp)
+	if !ok {
+		return nil, errors.Errorf("invalid expression type %T", exp)
+	}
+	m := map[string]struct{}{}
+	for _, e := range elementExpressions(je, exp) {
+		if err := appendJSONFuncCalls(e, m); err != nil {
+			return nil, err
+		}
+	}
+	arr := make([]string, 0, len(m))
+	for n := range m {
+		arr = append(arr, n)
+	}
+	return arr, nil
+}
+
+func appendJSONFuncCalls(exp hcl.Expression, m map[string]struct{}) error {
+	v := reflect.ValueOf(exp)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return errors.Errorf("invalid json expression kind %T %v", exp, v.Kind())
+	}
+	if v.Elem().Kind() != reflect.Struct {
+		return errors.Errorf("invalid json expression pointer to %T %v", exp, v.Elem().Kind())
+	}
+	src := v.Elem().FieldByName("src")
+	if src.IsZero() {
+		return errors.Errorf("%v has no property src", v.Elem().Type())
+	}
+	if src.Kind() != reflect.Interface {
+		return errors.Errorf("%v src is not interface: %v", src.Type(), src.Kind())
+	}
+	src = src.Elem()
+	if src.IsNil() {
+		return nil
+	}
+	if src.Kind() == reflect.Ptr {
+		src = src.Elem()
+	}
+	if src.Kind() != reflect.Struct {
+		return errors.Errorf("%v is not struct: %v", src.Type(), src.Kind())
+	}
+
+	// hcl/v2/json/ast#stringVal
+	val := src.FieldByName("Value")
+	if val.IsZero() {
+		return nil
+	}
+	rng := src.FieldByName("SrcRange")
+	if val.IsZero() {
+		return nil
+	}
+	var stringVal struct {
+		Value    string
+		SrcRange hcl.Range
+	}
+
+	if !val.Type().AssignableTo(reflect.ValueOf(stringVal.Value).Type()) {
+		return nil
+	}
+	if !rng.Type().AssignableTo(reflect.ValueOf(stringVal.SrcRange).Type()) {
+		return nil
+	}
+	// reflect.Set does not work for unexported fields
+	stringVal.Value = *(*string)(unsafe.Pointer(val.UnsafeAddr()))
+	stringVal.SrcRange = *(*hcl.Range)(unsafe.Pointer(rng.UnsafeAddr()))
+
+	expr, diags := hclsyntax.ParseExpression([]byte(stringVal.Value), stringVal.SrcRange.Filename, stringVal.SrcRange.Start)
+	if diags.HasErrors() {
+		return nil
+	}
+
+	fns, err := funcCalls(expr)
+	if err != nil {
+		return err
+	}
+
+	for _, fn := range fns {
+		m[fn] = struct{}{}
+	}
+
+	return nil
+}
+
+func funcCalls(exp hcl.Expression) ([]string, hcl.Diagnostics) {
+	node, ok := exp.(hclsyntax.Node)
+	if !ok {
+		fns, err := jsonFuncCallsRecursive(exp)
+		if err != nil {
+			return nil, hcl.Diagnostics{
+				&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid expression",
+					Detail:   err.Error(),
+					Subject:  exp.Range().Ptr(),
+					Context:  exp.Range().Ptr(),
+				},
+			}
+		}
+		return fns, nil
+	}
+
+	var funcnames []string
+	hcldiags := hclsyntax.VisitAll(node, func(n hclsyntax.Node) hcl.Diagnostics {
+		if fe, ok := n.(*hclsyntax.FunctionCallExpr); ok {
+			funcnames = append(funcnames, fe.Name)
+		}
+		return nil
+	})
+	if hcldiags.HasErrors() {
+		return nil, hcldiags
+	}
+	return funcnames, nil
+}
+
+func (sc *StaticConfig) loadDeps(exp hcl.Expression, exclude map[string]struct{}) hcl.Diagnostics {
+	fns, hcldiags := funcCalls(exp)
+	if hcldiags.HasErrors() {
+		return hcldiags
+	}
+
+	for _, fn := range fns {
+		if err := sc.resolveFunction(fn); err != nil {
+			return hcl.Diagnostics{
+				&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid expression",
+					Detail:   err.Error(),
+					Subject:  exp.Range().Ptr(),
+					Context:  exp.Range().Ptr(),
+				},
+			}
+		}
+	}
+
+	for _, v := range exp.Variables() {
+		if _, ok := exclude[v.RootName()]; ok {
+			continue
+		}
+		if err := sc.resolveValue(v.RootName()); err != nil {
+			return hcl.Diagnostics{
+				&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid expression",
+					Detail:   err.Error(),
+					Subject:  v.SourceRange().Ptr(),
+					Context:  v.SourceRange().Ptr(),
+				},
+			}
+		}
+	}
+
+	return nil
+}
+
+func (sc *StaticConfig) resolveFunction(name string) error {
+	if _, ok := sc.funcs[name]; ok {
+		return nil
+	}
+	f, ok := sc.funcDefs[name]
+	if !ok {
+		if _, ok := sc.ectx.Functions[name]; ok {
+			return nil
+		}
+		return errors.Errorf("undefined function %s", name)
+	}
+	if _, ok := sc.progressF[name]; ok {
+		return errors.Errorf("function cycle not allowed for %s", name)
+	}
+	sc.progressF[name] = struct{}{}
+
+	paramExprs, paramsDiags := hcl.ExprList(f.Params.Expr)
+	if paramsDiags.HasErrors() {
+		return paramsDiags
+	}
+	var diags hcl.Diagnostics
+	params := map[string]struct{}{}
+	for _, paramExpr := range paramExprs {
+		param := hcl.ExprAsKeyword(paramExpr)
+		if param == "" {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid param element",
+				Detail:   "Each parameter name must be an identifier.",
+				Subject:  paramExpr.Range().Ptr(),
+			})
+		}
+		params[param] = struct{}{}
+	}
+	var variadic hcl.Expression
+	if f.Variadic != nil {
+		variadic = f.Variadic.Expr
+		param := hcl.ExprAsKeyword(variadic)
+		if param == "" {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid param element",
+				Detail:   "Each parameter name must be an identifier.",
+				Subject:  f.Variadic.Range.Ptr(),
+			})
+		}
+		params[param] = struct{}{}
+	}
+	if diags.HasErrors() {
+		return diags
+	}
+
+	if diags := sc.loadDeps(f.Result.Expr, params); diags.HasErrors() {
+		return diags
+	}
+
+	v, diags := userfunc.NewFunction(f.Params.Expr, variadic, f.Result.Expr, func() *hcl.EvalContext {
+		return &sc.ectx
+	})
+	if diags.HasErrors() {
+		return diags
+	}
+	sc.funcs[name] = v
+	sc.ectx.Functions[name] = v
+
+	return nil
+}
+
+func (sc *StaticConfig) resolveValue(name string) (err error) {
+	if _, ok := sc.ectx.Variables[name]; ok {
+		return nil
 	}
 	if _, ok := sc.progress[name]; ok {
-		return nil, errors.Errorf("variable cycle not allowed")
+		return errors.Errorf("variable cycle not allowed for %s", name)
 	}
 	sc.progress[name] = struct{}{}
 
+	var v *cty.Value
 	defer func() {
 		if v != nil {
-			sc.values[name] = *v
+			sc.ectx.Variables[name] = *v
 		}
 	}()
 
@@ -207,43 +491,22 @@ func (sc *StaticConfig) resolveValue(name string) (v *cty.Value, err error) {
 	if !ok {
 		def, ok = sc.defaults[name]
 		if !ok {
-			return nil, errors.Errorf("undefined variable %q", name)
+			return errors.Errorf("undefined variable %q", name)
 		}
 	}
 
 	if def == nil {
-		v := cty.StringVal(sc.env[name])
-		return &v, nil
+		vv := cty.StringVal(sc.env[name])
+		v = &vv
+		return
 	}
 
-	ectx := &hcl.EvalContext{
-		Variables: map[string]cty.Value{},
-		Functions: stdlibFunctions, // user functions not possible atm
+	if diags := sc.loadDeps(def.Expr, nil); diags.HasErrors() {
+		return diags
 	}
-	for _, v := range def.Expr.Variables() {
-		value, err := sc.resolveValue(v.RootName())
-		if err != nil {
-			var diags hcl.Diagnostics
-			if !errors.As(err, &diags) {
-				return nil, err
-			}
-			r := v.SourceRange()
-			return nil, hcl.Diagnostics{
-				&hcl.Diagnostic{
-					Severity: hcl.DiagError,
-					Summary:  "Invalid expression",
-					Detail:   err.Error(),
-					Subject:  &r,
-					Context:  &r,
-				},
-			}
-		}
-		ectx.Variables[v.RootName()] = *value
-	}
-
-	vv, diags := def.Expr.Value(ectx)
+	vv, diags := def.Expr.Value(&sc.ectx)
 	if diags.HasErrors() {
-		return nil, diags
+		return diags
 	}
 
 	_, isVar := sc.defaults[name]
@@ -252,29 +515,33 @@ func (sc *StaticConfig) resolveValue(name string) (v *cty.Value, err error) {
 		if vv.Type().Equals(cty.Bool) {
 			b, err := strconv.ParseBool(envv)
 			if err != nil {
-				return nil, errors.Wrapf(err, "failed to parse %s as bool", name)
+				return errors.Wrapf(err, "failed to parse %s as bool", name)
 			}
-			v := cty.BoolVal(b)
-			return &v, nil
+			vv := cty.BoolVal(b)
+			v = &vv
+			return nil
 		} else if vv.Type().Equals(cty.String) {
-			v := cty.StringVal(envv)
-			return &v, nil
+			vv := cty.StringVal(envv)
+			v = &vv
+			return nil
 		} else if vv.Type().Equals(cty.Number) {
 			n, err := strconv.ParseFloat(envv, 64)
 			if err == nil && (math.IsNaN(n) || math.IsInf(n, 0)) {
 				err = errors.Errorf("invalid number value")
 			}
 			if err != nil {
-				return nil, errors.Wrapf(err, "failed to parse %s as number", name)
+				return errors.Wrapf(err, "failed to parse %s as number", name)
 			}
-			v := cty.NumberVal(big.NewFloat(n))
-			return &v, nil
+			vv := cty.NumberVal(big.NewFloat(n))
+			v = &vv
+			return nil
 		} else {
 			// TODO: support lists with csv values
-			return nil, errors.Errorf("unsupported type %s for variable %s", v.Type(), name)
+			return errors.Errorf("unsupported type %s for variable %s", v.Type(), name)
 		}
 	}
-	return &vv, nil
+	v = &vv
+	return nil
 }
 
 func ParseHCLFile(dt []byte, fn string) (*hcl.File, *StaticConfig, error) {
@@ -331,36 +598,11 @@ func parseHCLFile(dt []byte, fn string) (f *hcl.File, _ *StaticConfig, err error
 }
 
 func ParseHCL(b hcl.Body, sc *StaticConfig) (_ *Config, err error) {
-
-	// evaluate variables
-	variables, err := sc.Values(true)
+	ctx, err := sc.EvalContext(true)
 	if err != nil {
 		return nil, err
 	}
 
-	userFunctions, _, diags := userfunc.DecodeUserFunctions(b, "function", func() *hcl.EvalContext {
-		return &hcl.EvalContext{
-			Functions: stdlibFunctions,
-			Variables: variables,
-		}
-	})
-	if diags.HasErrors() {
-		return nil, diags
-	}
-
-	functions := make(map[string]function.Function)
-	for k, v := range stdlibFunctions {
-		functions[k] = v
-	}
-	for k, v := range userFunctions {
-		functions[k] = v
-	}
-
-	ctx := &hcl.EvalContext{
-		Variables: variables,
-		Functions: functions,
-	}
-
 	var c Config
 
 	// Decode with variables and functions.

bake/hcl_test.go

@@ -513,3 +513,56 @@ func TestJSONAttributes(t *testing.T) {
 	require.Equal(t, c.Targets[0].Name, "app")
 	require.Equal(t, "pre-abc-def", c.Targets[0].Args["v1"])
 }
+
+func TestJSONFunctions(t *testing.T) {
+	dt := []byte(`{
+	"FOO": "abc",
+	"function": {
+		"myfunc": {
+			"params": ["inp"],
+			"result": "<${upper(inp)}-${FOO}>"
+		}
+	},
+	"target": {
+		"app": {
+			"args": {
+				"v1": "pre-${myfunc(\"foo\")}"
+			}
+		}
+	}}`)
+
+	c, err := ParseFile(dt, "docker-bake.json")
+	require.NoError(t, err)
+
+	require.Equal(t, 1, len(c.Targets))
+	require.Equal(t, c.Targets[0].Name, "app")
+	require.Equal(t, "pre-<FOO-abc>", c.Targets[0].Args["v1"])
+}
+
+func TestHCLFunctionInAttr(t *testing.T) {
+	dt := []byte(`
+	function "brace" {
+		params = [inp]
+		result = "[${inp}]"
+	}
+	function "myupper" {
+		params = [val]
+		result = "${upper(val)} <> ${brace(v2)}"
+	}
+
+		v1=myupper("foo")
+		v2=lower("BAZ")
+		target "app" {
+			args = {
+				"v1": v1
+			}
+		}
+		`)
+
+	c, err := ParseFile(dt, "docker-bake.hcl")
+	require.NoError(t, err)
+
+	require.Equal(t, 1, len(c.Targets))
+	require.Equal(t, c.Targets[0].Name, "app")
+	require.Equal(t, "FOO <> [baz]", c.Targets[0].Args["v1"])
+}

util/userfunc/decode.go

@@ -2,7 +2,6 @@ package userfunc
 
 import (
 	"github.com/hashicorp/hcl/v2"
-	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/function"
 )
 
@@ -53,7 +52,7 @@ func decodeUserFunctions(body hcl.Body, blockType string, contextFunc ContextFun
 	}
 
 	funcs = make(map[string]function.Function)
-Blocks:
+
 	for _, block := range content.Blocks {
 		name := block.Labels[0]
 		funcContent, funcDiags := block.Body.Content(funcBodySchema)
@@ -68,88 +67,12 @@ Blocks:
 		if funcContent.Attributes["variadic_param"] != nil {
 			varParamExpr = funcContent.Attributes["variadic_param"].Expr
 		}
-
-		var params []string
-		var varParam string
-
-		paramExprs, paramsDiags := hcl.ExprList(paramsExpr)
-		diags = append(diags, paramsDiags...)
-		if paramsDiags.HasErrors() {
+		f, funcDiags := NewFunction(paramsExpr, varParamExpr, resultExpr, getBaseCtx)
+		if funcDiags.HasErrors() {
+			diags = append(diags, funcDiags...)
 			continue
 		}
-		for _, paramExpr := range paramExprs {
-			param := hcl.ExprAsKeyword(paramExpr)
-			if param == "" {
-				diags = append(diags, &hcl.Diagnostic{
-					Severity: hcl.DiagError,
-					Summary:  "Invalid param element",
-					Detail:   "Each parameter name must be an identifier.",
-					Subject:  paramExpr.Range().Ptr(),
-				})
-				continue Blocks
-			}
-			params = append(params, param)
-		}
-
-		if varParamExpr != nil {
-			varParam = hcl.ExprAsKeyword(varParamExpr)
-			if varParam == "" {
-				diags = append(diags, &hcl.Diagnostic{
-					Severity: hcl.DiagError,
-					Summary:  "Invalid variadic_param",
-					Detail:   "The variadic parameter name must be an identifier.",
-					Subject:  varParamExpr.Range().Ptr(),
-				})
-				continue
-			}
-		}
-
-		spec := &function.Spec{}
-		for _, paramName := range params {
-			spec.Params = append(spec.Params, function.Parameter{
-				Name: paramName,
-				Type: cty.DynamicPseudoType,
-			})
-		}
-		if varParamExpr != nil {
-			spec.VarParam = &function.Parameter{
-				Name: varParam,
-				Type: cty.DynamicPseudoType,
-			}
-		}
-		impl := func(args []cty.Value) (cty.Value, error) {
-			ctx := getBaseCtx()
-			ctx = ctx.NewChild()
-			ctx.Variables = make(map[string]cty.Value)
-
-			// The cty function machinery guarantees that we have at least
-			// enough args to fill all of our params.
-			for i, paramName := range params {
-				ctx.Variables[paramName] = args[i]
-			}
-			if spec.VarParam != nil {
-				varArgs := args[len(params):]
-				ctx.Variables[varParam] = cty.TupleVal(varArgs)
-			}
-
-			result, diags := resultExpr.Value(ctx)
-			if diags.HasErrors() {
-				// Smuggle the diagnostics out via the error channel, since
-				// a diagnostics sequence implements error. Caller can
-				// type-assert this to recover the individual diagnostics
-				// if desired.
-				return cty.DynamicVal, diags
-			}
-			return result, nil
-		}
-		spec.Type = func(args []cty.Value) (cty.Type, error) {
-			val, err := impl(args)
-			return val.Type(), err
-		}
-		spec.Impl = func(args []cty.Value, retType cty.Type) (cty.Value, error) {
-			return impl(args)
-		}
-		funcs[name] = function.New(spec)
+		funcs[name] = f
 	}
 
 	return funcs, remain, diags

util/userfunc/public.go

@@ -2,6 +2,7 @@ package userfunc
 
 import (
 	"github.com/hashicorp/hcl/v2"
+	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/function"
 )
 
@@ -40,3 +41,85 @@ type ContextFunc func() *hcl.EvalContext
 func DecodeUserFunctions(body hcl.Body, blockType string, context ContextFunc) (funcs map[string]function.Function, remain hcl.Body, diags hcl.Diagnostics) {
 	return decodeUserFunctions(body, blockType, context)
 }
+
+// NewFunction creates a new function instance from preparsed HCL expressions.
+func NewFunction(paramsExpr, varParamExpr, resultExpr hcl.Expression, getBaseCtx func() *hcl.EvalContext) (function.Function, hcl.Diagnostics) {
+	var params []string
+	var varParam string
+
+	paramExprs, paramsDiags := hcl.ExprList(paramsExpr)
+	if paramsDiags.HasErrors() {
+		return function.Function{}, paramsDiags
+	}
+	for _, paramExpr := range paramExprs {
+		param := hcl.ExprAsKeyword(paramExpr)
+		if param == "" {
+			return function.Function{}, hcl.Diagnostics{{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid param element",
+				Detail:   "Each parameter name must be an identifier.",
+				Subject:  paramExpr.Range().Ptr(),
+			}}
+		}
+		params = append(params, param)
+	}
+
+	if varParamExpr != nil {
+		varParam = hcl.ExprAsKeyword(varParamExpr)
+		if varParam == "" {
+			return function.Function{}, hcl.Diagnostics{{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid variadic_param",
+				Detail:   "The variadic parameter name must be an identifier.",
+				Subject:  varParamExpr.Range().Ptr(),
+			}}
+		}
+	}
+
+	spec := &function.Spec{}
+	for _, paramName := range params {
+		spec.Params = append(spec.Params, function.Parameter{
+			Name: paramName,
+			Type: cty.DynamicPseudoType,
+		})
+	}
+	if varParamExpr != nil {
+		spec.VarParam = &function.Parameter{
+			Name: varParam,
+			Type: cty.DynamicPseudoType,
+		}
+	}
+	impl := func(args []cty.Value) (cty.Value, error) {
+		ctx := getBaseCtx()
+		ctx = ctx.NewChild()
+		ctx.Variables = make(map[string]cty.Value)
+
+		// The cty function machinery guarantees that we have at least
+		// enough args to fill all of our params.
+		for i, paramName := range params {
+			ctx.Variables[paramName] = args[i]
+		}
+		if spec.VarParam != nil {
+			varArgs := args[len(params):]
+			ctx.Variables[varParam] = cty.TupleVal(varArgs)
+		}
+
+		result, diags := resultExpr.Value(ctx)
+		if diags.HasErrors() {
+			// Smuggle the diagnostics out via the error channel, since
+			// a diagnostics sequence implements error. Caller can
+			// type-assert this to recover the individual diagnostics
+			// if desired.
+			return cty.DynamicVal, diags
+		}
+		return result, nil
+	}
+	spec.Type = func(args []cty.Value) (cty.Type, error) {
+		val, err := impl(args)
+		return val.Type(), err
+	}
+	spec.Impl = func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		return impl(args)
+	}
+	return function.New(spec), nil
+}