ext/userfunc: use bare identifiers for param names

Now that we have the necessary functions to deal with this in the
low-level HCL API, it's more intuitive to use bare identifiers for these
parameter names. This reinforces the idea that they are symbols being
defined rather than arbitrary string expressions.

cmd/hcldec/spec-format.md

@@ -364,7 +364,7 @@ block type:
 
 ```
 function "add_one" {
-  params = ["n"]
+  params = [n]
   result = n + 1
 }
 ```
@@ -379,9 +379,15 @@ input file:
 
 ```
 function "upper" {
-  params = ["str"]
+  params = [str]
   result = upper(str)
 }
+
+function "min" {
+  params         = []
+  variadic_param = nums
+  result         = min(nums...)
+}
 ```
 
 Custom functions defined in the spec cannot be called from the spec itself.

ext/userfunc/README.md

@@ -7,9 +7,15 @@ Functions are defined via a specific block type, like this:
 
 ```hcl
 function "add" {
-  params = ["a", "b"]
+  params = [a, b]
   result = a + b
 }
+
+function "list" {
+  params         = []
+  variadic_param = items
+  result         = items
+}
 ```
 
 The extension is implemented as a pre-processor for `cty.Body` objects. Given

ext/userfunc/decode.go

@@ -1,7 +1,6 @@
 package userfunc
 
 import (
-	"github.com/hashicorp/hcl2/gohcl"
 	"github.com/hashicorp/hcl2/hcl"
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/function"
@@ -54,6 +53,7 @@ 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)
@@ -72,15 +72,34 @@ func decodeUserFunctions(body hcl.Body, blockType string, contextFunc ContextFun
 		var params []string
 		var varParam string
 
-		paramsDiags := gohcl.DecodeExpression(paramsExpr, nil, &params)
+		paramExprs, paramsDiags := hcl.ExprList(paramsExpr)
 		diags = append(diags, paramsDiags...)
 		if paramsDiags.HasErrors() {
 			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 {
-			paramsDiags := gohcl.DecodeExpression(varParamExpr, nil, &varParam)
-			diags = append(diags, paramsDiags...)
-			if paramsDiags.HasErrors() {
+			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
 			}
 		}

ext/userfunc/decode_test.go

@@ -4,8 +4,8 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 	"github.com/hashicorp/hcl2/hcl"
+	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -20,7 +20,7 @@ func TestDecodeUserFunctions(t *testing.T) {
 		{
 			`
 function "greet" {
-  params = ["name"]
+  params = [name]
   result = "Hello, ${name}."
 }
 `,
@@ -32,7 +32,7 @@ function "greet" {
 		{
 			`
 function "greet" {
-  params = ["name"]
+  params = [name]
   result = "Hello, ${name}."
 }
 `,
@@ -44,7 +44,7 @@ function "greet" {
 		{
 			`
 function "greet" {
-  params = ["name"]
+  params = [name]
   result = "Hello, ${name}."
 }
 `,
@@ -56,7 +56,7 @@ function "greet" {
 		{
 			`
 function "add" {
-  params = ["a", "b"]
+  params = [a, b]
   result = a + b
 }
 `,
@@ -69,7 +69,7 @@ function "add" {
 			`
 function "argstuple" {
   params = []
-  variadic_param = "args"
+  variadic_param = args
   result = args
 }
 `,
@@ -109,11 +109,11 @@ function "closure" {
 		{
 			`
 function "neg" {
-  params = ["val"]
+  params = [val]
   result = -val
 }
 function "add" {
-  params = ["a", "b"]
+  params = [a, b]
   result = a + b
 }
 `,
@@ -125,7 +125,7 @@ function "add" {
 		{
 			`
 function "neg" {
-  parrams = ["val"]
+  parrams = [val]
   result = -val
 }
 `,